diff --git a/bigchaindb/utils.py b/bigchaindb/utils.py
index 4dad9377..f87916b7 100644
--- a/bigchaindb/utils.py
+++ b/bigchaindb/utils.py
@@ -114,11 +114,16 @@ def condition_details_has_owner(condition_details, owner):
 
 
 def output_has_owner(output, owner):
-    if len(output['public_keys']) > 0:
+    # TODO
+    # Check whether it is really necessary to treat the single key case
+    # differently from the multiple keys case, and why not just use the same
+    # function for both cases.
+    if len(output['public_keys']) > 1:
         return condition_details_has_owner(
             output['condition']['details'], owner)
-    return False
-    # # TODO raise proper exception, e.g. invalid tx payload?
+    elif len(output['public_keys']) == 1:
+        return output['condition']['details']['public_key'] == owner
+    # TODO raise proper exception, e.g. invalid tx payload?
 
 
 def is_genesis_block(block):
diff --git a/bigchaindb/web/server.py b/bigchaindb/web/server.py
index 16c76b01..539d3f73 100644
--- a/bigchaindb/web/server.py
+++ b/bigchaindb/web/server.py
@@ -2,6 +2,7 @@
 
 The application is implemented in Flask and runs using Gunicorn.
 """
+
 import copy
 import multiprocessing
 
diff --git a/setup.py b/setup.py
index c6a5f63d..ce509518 100644
--- a/setup.py
+++ b/setup.py
@@ -71,7 +71,7 @@ install_requires = [
     'python-rapidjson==0.0.11',
     'logstats>=0.2.1',
     'flask>=0.10.1',
-    'flask-cors==2.1.2',
+    'flask-cors~=2.1.2',
     'flask-restful~=0.3.0',
     'requests~=2.9',
     'gunicorn~=19.0',