1 files changed, 4 insertions, 1 deletions
diff --git a/synapse/types.py b/synapse/types.py
index eebe29d1f0..e6afc05cee 100644
--- a/synapse/types.py
+++ b/synapse/types.py
@@ -16,6 +16,8 @@ import re
 import string
 from collections import namedtuple
 
+from six.moves import filter
+
 import attr
 
 from synapse.api.errors import SynapseError
@@ -240,7 +242,8 @@ def strip_invalid_mxid_characters(localpart):
     Returns:
         localpart (basestring): the localpart having been stripped
     """
-    return filter(lambda c: c in mxid_localpart_allowed_characters, localpart)
+    filtered = filter(lambda c: c in mxid_localpart_allowed_characters, localpart)
+    return "".join(filtered)
 
 
 UPPER_CASE_PATTERN = re.compile(b"[A-Z_]")