diff --git a/bigchaindb/backend/localmongodb/schema.py b/bigchaindb/backend/localmongodb/schema.py
index 25ef7f4e..0607c883 100644
--- a/bigchaindb/backend/localmongodb/schema.py
+++ b/bigchaindb/backend/localmongodb/schema.py
@@ -59,6 +59,7 @@ def create_transactions_secondary_index(conn, dbname):
 
     # to query the transactions for a transaction id, this field is unique
     conn.conn[dbname]['transactions'].create_index('id',
+                                                   unique=True,
                                                    name='transaction_id')
 
     # secondary index for asset uuid, this field is unique
@@ -93,7 +94,7 @@ def create_assets_secondary_index(conn, dbname):
 
 def create_blocks_secondary_index(conn, dbname):
     conn.conn[dbname]['blocks']\
-        .create_index([('height', DESCENDING)], name='height')
+        .create_index([('height', DESCENDING)], name='height', unique=True)
 
 
 def create_metadata_secondary_index(conn, dbname):
diff --git a/tests/backend/localmongodb/test_queries.py b/tests/backend/localmongodb/test_queries.py
index 363d02a1..51e20265 100644
--- a/tests/backend/localmongodb/test_queries.py
+++ b/tests/backend/localmongodb/test_queries.py
@@ -218,9 +218,9 @@ def test_get_spending_transactions(user_pk, user_sk):
     tx1 = Transaction.create([user_pk], out * 3)
     tx1.sign([user_sk])
     inputs = tx1.to_inputs()
-    tx2 = Transaction.transfer([inputs[0]], out, tx1.id)
-    tx3 = Transaction.transfer([inputs[1]], out, tx1.id)
-    tx4 = Transaction.transfer([inputs[2]], out, tx1.id)
+    tx2 = Transaction.transfer([inputs[0]], out, tx1.id).sign([user_sk])
+    tx3 = Transaction.transfer([inputs[1]], out, tx1.id).sign([user_sk])
+    tx4 = Transaction.transfer([inputs[2]], out, tx1.id).sign([user_sk])
     txns = [tx.to_dict() for tx in [tx1, tx2, tx3, tx4]]
     conn.db.transactions.insert_many(txns)
 
diff --git a/tests/backend/localmongodb/test_schema.py b/tests/backend/localmongodb/test_schema.py
index b9b22639..136715ab 100644
--- a/tests/backend/localmongodb/test_schema.py
+++ b/tests/backend/localmongodb/test_schema.py
@@ -99,12 +99,16 @@ def test_create_secondary_indexes():
     indexes = conn.conn[dbname]['assets'].index_information().keys()
     assert set(indexes) == {'_id_', 'asset_id', 'text'}
 
-    indexes = conn.conn[dbname]['transactions'].index_information().keys()
+    index_info = conn.conn[dbname]['transactions'].index_information()
+    indexes = index_info.keys()
     assert set(indexes) == {
             '_id_', 'transaction_id', 'asset_id', 'outputs', 'inputs'}
+    assert index_info['transaction_id']['unique']
 
-    indexes = conn.conn[dbname]['blocks'].index_information().keys()
+    index_info = conn.conn[dbname]['blocks'].index_information()
+    indexes = index_info.keys()
     assert set(indexes) == {'_id_', 'height'}
+    assert index_info['height']['unique']
 
     index_info = conn.conn[dbname]['utxos'].index_information()
     assert set(index_info.keys()) == {'_id_', 'utxo'}