diff --git a/src/authorization/AcpReader.ts b/src/authorization/AcpReader.ts
index 3d1d6d7fe..e9b842147 100644
--- a/src/authorization/AcpReader.ts
+++ b/src/authorization/AcpReader.ts
@@ -15,6 +15,7 @@ import { InternalServerError } from '../util/errors/InternalServerError';
 import { NotFoundHttpError } from '../util/errors/NotFoundHttpError';
 import type { IdentifierStrategy } from '../util/identifiers/IdentifierStrategy';
 import { IdentifierMap } from '../util/map/IdentifierMap';
+import { getDefault } from '../util/map/MapUtil';
 import { readableToQuads } from '../util/StreamUtil';
 import { ACL } from '../util/Vocabularies';
 import { getAccessControlledResources } from './AcpUtil';
@@ -78,12 +79,8 @@ export class AcpReader extends PermissionReader {
     // Extract all the policies relevant for the target
     const identifiers = this.getAncestorIdentifiers(target);
     for (const identifier of identifiers) {
-      let acrs = resourceCache.get(identifier);
-      if (!acrs) {
-        const data = await this.readAcrData(identifier);
-        acrs = [ ...getAccessControlledResources(data) ];
-        resourceCache.set(identifier, acrs);
-      }
+      const acrs = await getDefault(resourceCache, identifier, async(): Promise<IAccessControlledResource[]> =>
+        [ ...getAccessControlledResources(await this.readAcrData(identifier)) ]);
       const size = policies.length;
       policies.push(...this.getEffectivePolicies(target, acrs));
       this.logger.debug(`Found ${policies.length - size} policies relevant for ${target.path} in ${identifier.path}`);
diff --git a/src/authorization/PathBasedReader.ts b/src/authorization/PathBasedReader.ts
index 6d858a1b5..533425d26 100644
--- a/src/authorization/PathBasedReader.ts
+++ b/src/authorization/PathBasedReader.ts
@@ -1,6 +1,7 @@
 import { getLoggerFor } from '../logging/LogUtil';
 import { concat } from '../util/IterableUtil';
 import { IdentifierMap, IdentifierSetMultiMap } from '../util/map/IdentifierMap';
+import { getDefault } from '../util/map/MapUtil';
 import { ensureTrailingSlash, trimTrailingSlashes } from '../util/PathUtil';
 import type { PermissionReaderInput } from './PermissionReader';
 import { PermissionReader } from './PermissionReader';
@@ -44,11 +45,7 @@ export class PathBasedReader extends PermissionReader {
     for (const [ identifier, modes ] of accessMap) {
       const reader = this.findReader(identifier.path);
       if (reader) {
-        let matches = result.get(reader);
-        if (!matches) {
-          matches = new IdentifierSetMultiMap();
-          result.set(reader, matches);
-        }
+        const matches = getDefault(result, reader, (): AccessMap => new IdentifierSetMultiMap());
         matches.set(identifier, modes);
       }
     }
diff --git a/src/authorization/UnionPermissionReader.ts b/src/authorization/UnionPermissionReader.ts
index ad35b7977..7972d9ff3 100644
--- a/src/authorization/UnionPermissionReader.ts
+++ b/src/authorization/UnionPermissionReader.ts
@@ -27,7 +27,7 @@ export class UnionPermissionReader extends UnionHandler<PermissionReader> {
   private mergePermissionMaps(permissionMap: PermissionMap, result: PermissionMap): void {
     for (const [ identifier, permissionSet ] of permissionMap) {
       for (const [ credential, permission ] of Object.entries(permissionSet) as [keyof PermissionSet, Permission][]) {
-        const resultSet = getDefault(result, identifier, {});
+        const resultSet = getDefault(result, identifier, (): PermissionSet => ({}));
         resultSet[credential] = this.mergePermissions(permission, resultSet[credential]);
       }
     }
diff --git a/src/storage/conversion/BaseTypedRepresentationConverter.ts b/src/storage/conversion/BaseTypedRepresentationConverter.ts
index e5e3c40b6..b4f3c3d2d 100644
--- a/src/storage/conversion/BaseTypedRepresentationConverter.ts
+++ b/src/storage/conversion/BaseTypedRepresentationConverter.ts
@@ -1,10 +1,10 @@
 import type { ValuePreferences } from '../../http/representation/RepresentationPreferences';
 import { NotImplementedHttpError } from '../../util/errors/NotImplementedHttpError';
+import type { PromiseOrValue } from '../../util/PromiseUtil';
 import { getConversionTarget, getTypeWeight, preferencesToString } from './ConversionUtil';
 import type { RepresentationConverterArgs } from './RepresentationConverter';
 import { TypedRepresentationConverter } from './TypedRepresentationConverter';
 
-type PromiseOrValue<T> = T | Promise<T>;
 type ValuePreferencesArg =
   PromiseOrValue<string> |
   PromiseOrValue<string[]> |
diff --git a/src/util/PromiseUtil.ts b/src/util/PromiseUtil.ts
index 8d38b3ae9..a8cb1609d 100644
--- a/src/util/PromiseUtil.ts
+++ b/src/util/PromiseUtil.ts
@@ -1,5 +1,29 @@
+import { types } from 'util';
 import { createAggregateError } from './errors/HttpErrorUtil';
 
+export type PromiseOrValue<T> = T | Promise<T>;
+
+/**
+ * Verifies if the given value is a Promise or not.
+ * @param object - Object to check.
+ */
+export function isPromise<T>(object: PromiseOrValue<T>): object is Promise<T> {
+  return types.isPromise(object);
+}
+
+/**
+ * Calls `callback` with the resolved value of `object`.
+ * In case `object` is a Promise, the result will also be a Promise,
+ * otherwise the result will be sync.
+ */
+export function resolvePromiseOrValue<TIn, TOut>(object: PromiseOrValue<TIn>, callback: (val: TIn) => TOut):
+PromiseOrValue<TOut> {
+  if (isPromise(object)) {
+    return object.then((val): TOut => callback(val));
+  }
+  return callback(object);
+}
+
 // eslint-disable-next-line @typescript-eslint/no-empty-function
 function noop(): void {}
 
diff --git a/src/util/StreamUtil.ts b/src/util/StreamUtil.ts
index 306ec4eff..0831e0495 100644
--- a/src/util/StreamUtil.ts
+++ b/src/util/StreamUtil.ts
@@ -10,6 +10,7 @@ import { isHttpRequest } from '../server/HttpRequest';
 import { InternalServerError } from './errors/InternalServerError';
 import type { Guarded } from './GuardedStream';
 import { guardStream } from './GuardedStream';
+import type { PromiseOrValue } from './PromiseUtil';
 
 export const endOfStream = promisify(eos);
 
@@ -119,12 +120,12 @@ export interface AsyncTransformOptions<T = any> extends DuplexOptions {
   /**
    * Transforms data from the source by calling the `push` method
    */
-  transform?: (this: Transform, data: T, encoding: string) => any | Promise<any>;
+  transform?: (this: Transform, data: T, encoding: string) => PromiseOrValue<any>;
 
   /**
    * Performs any final actions after the source has ended
    */
-  flush?: (this: Transform) => any | Promise<any>;
+  flush?: (this: Transform) => PromiseOrValue<any>;
 }
 
 /**
diff --git a/src/util/locking/EqualReadWriteLocker.ts b/src/util/locking/EqualReadWriteLocker.ts
index d57b554c7..e381cd1e0 100644
--- a/src/util/locking/EqualReadWriteLocker.ts
+++ b/src/util/locking/EqualReadWriteLocker.ts
@@ -1,4 +1,5 @@
 import type { ResourceIdentifier } from '../../http/representation/ResourceIdentifier';
+import type { PromiseOrValue } from '../PromiseUtil';
 import type { ReadWriteLocker } from './ReadWriteLocker';
 import type { ResourceLocker } from './ResourceLocker';
 
@@ -12,11 +13,11 @@ export class EqualReadWriteLocker implements ReadWriteLocker {
     this.locker = locker;
   }
 
-  public async withReadLock<T>(identifier: ResourceIdentifier, whileLocked: () => (Promise<T> | T)): Promise<T> {
+  public async withReadLock<T>(identifier: ResourceIdentifier, whileLocked: () => PromiseOrValue<T>): Promise<T> {
     return this.withLock(identifier, whileLocked);
   }
 
-  public async withWriteLock<T>(identifier: ResourceIdentifier, whileLocked: () => (Promise<T> | T)): Promise<T> {
+  public async withWriteLock<T>(identifier: ResourceIdentifier, whileLocked: () => PromiseOrValue<T>): Promise<T> {
     return this.withLock(identifier, whileLocked);
   }
 
@@ -26,7 +27,7 @@ export class EqualReadWriteLocker implements ReadWriteLocker {
    * @param identifier - Identifier of resource that needs to be locked.
    * @param whileLocked - Function to resolve while the resource is locked.
    */
-  private async withLock<T>(identifier: ResourceIdentifier, whileLocked: () => T | Promise<T>): Promise<T> {
+  private async withLock<T>(identifier: ResourceIdentifier, whileLocked: () => PromiseOrValue<T>): Promise<T> {
     await this.locker.acquire(identifier);
     try {
       return await whileLocked();
diff --git a/src/util/locking/ExpiringReadWriteLocker.ts b/src/util/locking/ExpiringReadWriteLocker.ts
index 249dc8de0..fb9356e62 100644
--- a/src/util/locking/ExpiringReadWriteLocker.ts
+++ b/src/util/locking/ExpiringReadWriteLocker.ts
@@ -1,4 +1,5 @@
 import type { ResourceIdentifier } from '../../http/representation/ResourceIdentifier';
+import type { PromiseOrValue } from '../PromiseUtil';
 import type { ReadWriteLocker } from './ReadWriteLocker';
 
 /**
@@ -14,7 +15,7 @@ export interface ExpiringReadWriteLocker extends ReadWriteLocker {
    * @param whileLocked - A function to execute while the resource is locked.
    * Receives a callback as input parameter to maintain the lock.
    */
-  withReadLock: <T>(identifier: ResourceIdentifier, whileLocked: (maintainLock: () => void) => T | Promise<T>)
+  withReadLock: <T>(identifier: ResourceIdentifier, whileLocked: (maintainLock: () => void) => PromiseOrValue<T>)
   => Promise<T>;
 
   /**
@@ -26,6 +27,6 @@ export interface ExpiringReadWriteLocker extends ReadWriteLocker {
    * @param whileLocked - A function to execute while the resource is locked.
    * Receives a callback as input parameter to maintain the lock.
    */
-  withWriteLock: <T>(identifier: ResourceIdentifier, whileLocked: (maintainLock: () => void) => T | Promise<T>)
+  withWriteLock: <T>(identifier: ResourceIdentifier, whileLocked: (maintainLock: () => void) => PromiseOrValue<T>)
   => Promise<T>;
 }
diff --git a/src/util/locking/GreedyReadWriteLocker.ts b/src/util/locking/GreedyReadWriteLocker.ts
index 86abfd0d5..5023db4e8 100644
--- a/src/util/locking/GreedyReadWriteLocker.ts
+++ b/src/util/locking/GreedyReadWriteLocker.ts
@@ -2,6 +2,7 @@ import type { ResourceIdentifier } from '../../http/representation/ResourceIdent
 import type { KeyValueStorage } from '../../storage/keyvalue/KeyValueStorage';
 import { ForbiddenHttpError } from '../errors/ForbiddenHttpError';
 import { InternalServerError } from '../errors/InternalServerError';
+import type { PromiseOrValue } from '../PromiseUtil';
 import type { ReadWriteLocker } from './ReadWriteLocker';
 import type { ResourceLocker } from './ResourceLocker';
 
@@ -43,7 +44,7 @@ export class GreedyReadWriteLocker implements ReadWriteLocker {
     this.suffixes = suffixes;
   }
 
-  public async withReadLock<T>(identifier: ResourceIdentifier, whileLocked: () => (Promise<T> | T)): Promise<T> {
+  public async withReadLock<T>(identifier: ResourceIdentifier, whileLocked: () => PromiseOrValue<T>): Promise<T> {
     await this.preReadSetup(identifier);
     try {
       return await whileLocked();
@@ -52,7 +53,7 @@ export class GreedyReadWriteLocker implements ReadWriteLocker {
     }
   }
 
-  public async withWriteLock<T>(identifier: ResourceIdentifier, whileLocked: () => (Promise<T> | T)): Promise<T> {
+  public async withWriteLock<T>(identifier: ResourceIdentifier, whileLocked: () => PromiseOrValue<T>): Promise<T> {
     if (identifier.path.endsWith(`.${this.suffixes.count}`)) {
       throw new ForbiddenHttpError('This resource is used for internal purposes.');
     }
@@ -117,7 +118,7 @@ export class GreedyReadWriteLocker implements ReadWriteLocker {
   /**
    * Safely runs an action on the count.
    */
-  private async withInternalReadLock<T>(identifier: ResourceIdentifier, whileLocked: () => (Promise<T> | T)):
+  private async withInternalReadLock<T>(identifier: ResourceIdentifier, whileLocked: () => PromiseOrValue<T>):
   Promise<T> {
     const read = this.getReadLockKey(identifier);
     await this.locker.acquire(read);
diff --git a/src/util/locking/ReadWriteLocker.ts b/src/util/locking/ReadWriteLocker.ts
index eee10b3e3..8fc0a9b49 100644
--- a/src/util/locking/ReadWriteLocker.ts
+++ b/src/util/locking/ReadWriteLocker.ts
@@ -1,4 +1,5 @@
 import type { ResourceIdentifier } from '../../http/representation/ResourceIdentifier';
+import type { PromiseOrValue } from '../PromiseUtil';
 
 /**
  * Allows the locking of resources which is needed for non-atomic {@link ResourceStore}s.
@@ -14,7 +15,7 @@ export interface ReadWriteLocker {
    *
    * @returns A promise resolving when the lock is released.
    */
-  withReadLock: <T>(identifier: ResourceIdentifier, whileLocked: () => T | Promise<T>) => Promise<T>;
+  withReadLock: <T>(identifier: ResourceIdentifier, whileLocked: () => PromiseOrValue<T>) => Promise<T>;
 
   /**
    * Run the given function while the resource is locked.
@@ -26,5 +27,5 @@ export interface ReadWriteLocker {
    *
    * @returns A promise resolving when the lock is released.
    */
-  withWriteLock: <T>(identifier: ResourceIdentifier, whileLocked: () => T | Promise<T>) => Promise<T>;
+  withWriteLock: <T>(identifier: ResourceIdentifier, whileLocked: () => PromiseOrValue<T>) => Promise<T>;
 }
diff --git a/src/util/locking/RedisLocker.ts b/src/util/locking/RedisLocker.ts
index d2bff1bfc..5d0843fa8 100644
--- a/src/util/locking/RedisLocker.ts
+++ b/src/util/locking/RedisLocker.ts
@@ -5,6 +5,7 @@ import type { Initializable } from '../../init/Initializable';
 import { getLoggerFor } from '../../logging/LogUtil';
 import type { AttemptSettings } from '../LockUtils';
 import { retryFunction } from '../LockUtils';
+import type { PromiseOrValue } from '../PromiseUtil';
 import type { ReadWriteLocker } from './ReadWriteLocker';
 import type { ResourceLocker } from './ResourceLocker';
 import type { RedisResourceLock, RedisReadWriteLock, RedisAnswer } from './scripts/RedisLuaScripts';
@@ -127,7 +128,7 @@ export class RedisLocker implements ReadWriteLocker, ResourceLocker, Initializab
     };
   }
 
-  public async withReadLock<T>(identifier: ResourceIdentifier, whileLocked: () => (Promise<T> | T)): Promise<T> {
+  public async withReadLock<T>(identifier: ResourceIdentifier, whileLocked: () => PromiseOrValue<T>): Promise<T> {
     const key = this.getReadWriteKey(identifier);
     await retryFunction(
       this.swallowFalse(this.redisRw.acquireReadLock.bind(this.redisRw, key)),
@@ -143,7 +144,7 @@ export class RedisLocker implements ReadWriteLocker, ResourceLocker, Initializab
     }
   }
 
-  public async withWriteLock<T>(identifier: ResourceIdentifier, whileLocked: () => (Promise<T> | T)): Promise<T> {
+  public async withWriteLock<T>(identifier: ResourceIdentifier, whileLocked: () => PromiseOrValue<T>): Promise<T> {
     const key = this.getReadWriteKey(identifier);
     await retryFunction(
       this.swallowFalse(this.redisRw.acquireWriteLock.bind(this.redisRw, key)),
diff --git a/src/util/locking/VoidLocker.ts b/src/util/locking/VoidLocker.ts
index bc79f792e..2d976737d 100644
--- a/src/util/locking/VoidLocker.ts
+++ b/src/util/locking/VoidLocker.ts
@@ -1,5 +1,6 @@
 import type { ResourceIdentifier } from '../../http/representation/ResourceIdentifier';
 import { getLoggerFor } from '../../logging/LogUtil';
+import type { PromiseOrValue } from '../PromiseUtil';
 import type { ExpiringReadWriteLocker } from './ExpiringReadWriteLocker';
 
 /**
@@ -20,14 +21,14 @@ export class VoidLocker implements ExpiringReadWriteLocker {
 
   public async withReadLock<T>(
     identifier: ResourceIdentifier,
-    whileLocked: (maintainLock: () => void) => T | Promise<T>,
+    whileLocked: (maintainLock: () => void) => PromiseOrValue<T>,
   ): Promise<T> {
     return whileLocked(noop);
   }
 
   public async withWriteLock<T>(
     identifier: ResourceIdentifier,
-    whileLocked: (maintainLock: () => void) => T | Promise<T>,
+    whileLocked: (maintainLock: () => void) => PromiseOrValue<T>,
   ): Promise<T> {
     return whileLocked(noop);
   }
diff --git a/src/util/locking/WrappedExpiringReadWriteLocker.ts b/src/util/locking/WrappedExpiringReadWriteLocker.ts
index 6bac5c8be..613f882fd 100644
--- a/src/util/locking/WrappedExpiringReadWriteLocker.ts
+++ b/src/util/locking/WrappedExpiringReadWriteLocker.ts
@@ -1,6 +1,7 @@
 import type { ResourceIdentifier } from '../../http/representation/ResourceIdentifier';
 import { getLoggerFor } from '../../logging/LogUtil';
 import { InternalServerError } from '../errors/InternalServerError';
+import type { PromiseOrValue } from '../PromiseUtil';
 import type { ExpiringReadWriteLocker } from './ExpiringReadWriteLocker';
 import type { ReadWriteLocker } from './ReadWriteLocker';
 import Timeout = NodeJS.Timeout;
@@ -24,12 +25,12 @@ export class WrappedExpiringReadWriteLocker implements ExpiringReadWriteLocker {
   }
 
   public async withReadLock<T>(identifier: ResourceIdentifier,
-    whileLocked: (maintainLock: () => void) => T | Promise<T>): Promise<T> {
+    whileLocked: (maintainLock: () => void) => PromiseOrValue<T>): Promise<T> {
     return this.locker.withReadLock(identifier, async(): Promise<T> => this.expiringPromise(identifier, whileLocked));
   }
 
   public async withWriteLock<T>(identifier: ResourceIdentifier,
-    whileLocked: (maintainLock: () => void) => T | Promise<T>): Promise<T> {
+    whileLocked: (maintainLock: () => void) => PromiseOrValue<T>): Promise<T> {
     return this.locker.withWriteLock(identifier, async(): Promise<T> => this.expiringPromise(identifier, whileLocked));
   }
 
@@ -39,7 +40,7 @@ export class WrappedExpiringReadWriteLocker implements ExpiringReadWriteLocker {
    * it receives. The ResourceIdentifier is only used for logging.
    */
   private async expiringPromise<T>(identifier: ResourceIdentifier,
-    whileLocked: (maintainLock: () => void) => T | Promise<T>): Promise<T> {
+    whileLocked: (maintainLock: () => void) => PromiseOrValue<T>): Promise<T> {
     let timer: Timeout;
     let createTimeout: () => Timeout;
 
diff --git a/src/util/map/MapUtil.ts b/src/util/map/MapUtil.ts
index 07d9af6ca..b26ee8939 100644
--- a/src/util/map/MapUtil.ts
+++ b/src/util/map/MapUtil.ts
@@ -1,3 +1,5 @@
+import { resolvePromiseOrValue } from '../PromiseUtil';
+import type { PromiseOrValue } from '../PromiseUtil';
 import type { SetMultiMap } from './SetMultiMap';
 
 export type MapKey<T> = T extends Map<infer TKey, any> ? TKey : never;
@@ -42,18 +44,33 @@ export function modify<T extends SetMultiMap<any, any>>(map: T, options: ModifyO
 
 /**
  * Finds the result of calling `map.get(key)`.
- * If there is no result, it instead returns the default value.
+ * If there is no result, it instead returns the result of the default function.
  * The Map will also be updated to assign that default value to the given key.
  *
  * @param map - Map to use.
  * @param key - Key to find the value for.
- * @param defaultValue - Value to insert and return if no result was found.
+ * @param defaultFn - Function to generate default value to insert and return if no result was found.
  */
-export function getDefault<TKey, TValue>(map: Map<TKey, TValue>, key: TKey, defaultValue: TValue): TValue {
+export function getDefault<TKey, TValue>(map: Map<TKey, TValue>, key: TKey, defaultFn: () => TValue): TValue;
+/**
+ * Finds the result of calling `map.get(key)`.
+ * If there is no result, it instead returns the result of the default function.
+ * The Map will also be updated to assign the resolved default value to the given key.
+ *
+ * @param map - Map to use.
+ * @param key - Key to find the value for.
+ * @param defaultFn - Function to generate default value to insert and return if no result was found.
+ */
+export function getDefault<TKey, TValue>(map: Map<TKey, TValue>, key: TKey, defaultFn: () => Promise<TValue>):
+Promise<TValue>;
+export function getDefault<TKey, TValue>(map: Map<TKey, TValue>, key: TKey, defaultFn: () => PromiseOrValue<TValue>):
+PromiseOrValue<TValue> {
   const value = map.get(key);
   if (value) {
     return value;
   }
-  map.set(key, defaultValue);
-  return defaultValue;
+  return resolvePromiseOrValue<TValue, TValue>(defaultFn(), (val): TValue => {
+    map.set(key, val);
+    return val;
+  });
 }
diff --git a/test/unit/storage/LockingResourceStore.test.ts b/test/unit/storage/LockingResourceStore.test.ts
index 730eccfe0..009d2b74b 100644
--- a/test/unit/storage/LockingResourceStore.test.ts
+++ b/test/unit/storage/LockingResourceStore.test.ts
@@ -6,6 +6,7 @@ import type { ResourceIdentifier } from '../../../src/http/representation/Resour
 import { LockingResourceStore } from '../../../src/storage/LockingResourceStore';
 import type { ResourceStore } from '../../../src/storage/ResourceStore';
 import type { ExpiringReadWriteLocker } from '../../../src/util/locking/ExpiringReadWriteLocker';
+import type { PromiseOrValue } from '../../../src/util/PromiseUtil';
 import { guardedStreamFrom } from '../../../src/util/StreamUtil';
 import { flushPromises } from '../../util/Util';
 
@@ -47,7 +48,7 @@ describe('A LockingResourceStore', (): void => {
 
     locker = {
       withReadLock: jest.fn(async <T>(id: ResourceIdentifier,
-        whileLocked: (maintainLock: () => void) => T | Promise<T>): Promise<T> => {
+        whileLocked: (maintainLock: () => void) => PromiseOrValue<T>): Promise<T> => {
         order.push('lock read');
         try {
           // Allows simulating a timeout event
@@ -61,7 +62,7 @@ describe('A LockingResourceStore', (): void => {
         }
       }),
       withWriteLock: jest.fn(async <T>(identifier: ResourceIdentifier,
-        whileLocked: (maintainLock: () => void) => T | Promise<T>): Promise<T> => {
+        whileLocked: (maintainLock: () => void) => PromiseOrValue<T>): Promise<T> => {
         order.push('lock write');
         try {
           return await whileLocked(emptyFn);
diff --git a/test/unit/util/locking/WrappedExpiringReadWriteLocker.test.ts b/test/unit/util/locking/WrappedExpiringReadWriteLocker.test.ts
index 05602e425..9f98bc447 100644
--- a/test/unit/util/locking/WrappedExpiringReadWriteLocker.test.ts
+++ b/test/unit/util/locking/WrappedExpiringReadWriteLocker.test.ts
@@ -1,6 +1,7 @@
 import type { ResourceIdentifier } from '../../../../src/http/representation/ResourceIdentifier';
 import type { ReadWriteLocker } from '../../../../src/util/locking/ReadWriteLocker';
 import { WrappedExpiringReadWriteLocker } from '../../../../src/util/locking/WrappedExpiringReadWriteLocker';
+import type { PromiseOrValue } from '../../../../src/util/PromiseUtil';
 
 jest.useFakeTimers();
 
@@ -14,9 +15,9 @@ describe('A WrappedExpiringReadWriteLocker', (): void => {
 
   beforeEach(async(): Promise<void> => {
     wrappedLocker = {
-      withReadLock: jest.fn(async<T>(id: ResourceIdentifier, whileLocked: () => T | Promise<T>):
+      withReadLock: jest.fn(async<T>(id: ResourceIdentifier, whileLocked: () => PromiseOrValue<T>):
       Promise<T> => whileLocked()),
-      withWriteLock: jest.fn(async<T>(id: ResourceIdentifier, whileLocked: () => T | Promise<T>):
+      withWriteLock: jest.fn(async<T>(id: ResourceIdentifier, whileLocked: () => PromiseOrValue<T>):
       Promise<T> => whileLocked()),
     };
 
diff --git a/test/unit/util/map/MapUtil.test.ts b/test/unit/util/map/MapUtil.test.ts
index d9fcc808e..4d652e433 100644
--- a/test/unit/util/map/MapUtil.test.ts
+++ b/test/unit/util/map/MapUtil.test.ts
@@ -44,12 +44,17 @@ describe('MapUtil', (): void => {
   describe('#getDefault', (): void => {
     it('returns the value it finds in the Map for the given key.', async(): Promise<void> => {
       const map = new Map([[ key1, 123 ]]);
-      expect(getDefault(map, key1, 999)).toBe(123);
+      expect(getDefault(map, key1, (): number => 999)).toBe(123);
     });
 
     it('returns the default value if it finds no value for the given key.', async(): Promise<void> => {
       const map = new Map([[ key1, 123 ]]);
-      expect(getDefault(map, key2, 999)).toBe(999);
+      expect(getDefault(map, key2, (): number => 999)).toBe(999);
+    });
+
+    it('can handle async default functions.', async(): Promise<void> => {
+      const map = new Map([[ key1, 123 ]]);
+      await expect(getDefault(map, key2, async(): Promise<number> => 999)).resolves.toBe(999);
     });
   });
 });