diff --git a/oauth2client/django_orm.py b/oauth2client/django_orm.py
index 6877208..f9ce26d 100644
--- a/oauth2client/django_orm.py
+++ b/oauth2client/django_orm.py
@@ -35,13 +35,15 @@ class CredentialsField(models.Field):
     return "TextField"
 
   def to_python(self, value):
-    if not value:
+    if value is None:
       return None
     if isinstance(value, oauth2client.client.Credentials):
       return value
     return pickle.loads(base64.b64decode(value))
 
   def get_db_prep_value(self, value, connection, prepared=False):
+    if value is None:
+      return None
     return base64.b64encode(pickle.dumps(value))
 
 
@@ -60,6 +62,8 @@ class FlowField(models.Field):
     return pickle.loads(base64.b64decode(value))
 
   def get_db_prep_value(self, value, connection, prepared=False):
+    if value is None:
+      return None
     return base64.b64encode(pickle.dumps(value))