diff --git a/synapse/handlers/directory.py b/synapse/handlers/directory.py
index ed9b0f8551..05e5c6ecfc 100644
--- a/synapse/handlers/directory.py
+++ b/synapse/handlers/directory.py
@@ -128,7 +128,7 @@ class DirectoryHandler(BaseHandler):
                 "servers": result.servers,
             })
         else:
-            raise SynapseError(404, "Room alias \"%s\" not found", room_alias)
+            raise SynapseError(404, "Room alias \"%s\" not found" % (room_alias,))
 
 
     @defer.inlineCallbacks