diff --git a/synapse/api/filtering.py b/synapse/api/filtering.py
index 583daea906..c8c55bc061 100644
--- a/synapse/api/filtering.py
+++ b/synapse/api/filtering.py
@@ -12,11 +12,13 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+
 from synapse.api.errors import SynapseError
 from synapse.storage.presence import UserPresenceState
 from synapse.types import UserID, RoomID
 from twisted.internet import defer
 
+import six
 import ujson as json
 import jsonschema
 from jsonschema import FormatChecker
@@ -353,10 +355,8 @@ class Filter(object):
             ev_type = event.get("type", None)
 
             content = event.get("content", {})
-            is_url = (
-                "url" in content
-                and isinstance(content["url"], basestring)
-            )
+            # check if there is a string url field in the content for filtering purposes
+            is_url = isinstance(content.get("url"), six.text_type)
 
         return self.check_fields(
             room_id,