--- a/src/plugins/plugin_xep_0045.py	Sun Mar 05 23:41:10 2017 +0100
+++ b/src/plugins/plugin_xep_0045.py	Mon Mar 06 21:22:59 2017 +0100
@@ -421,7 +421,9 @@
             try:
                 room_jid = jid.JID(room_jid_s)
             except (RuntimeError, jid.InvalidFormat, AttributeError):
-                return defer.fail(jid.InvalidFormat(_(u"Invalid room identifier: '%s'. Please give a room short or full identifier like 'room' or 'room@%s'.") % (room_jid_s, unicode(muc_service))))
+                return defer.fail(jid.InvalidFormat(_(u"Invalid room identifier: {room_id}'. Please give a room short or full identifier like 'room' or 'room@{muc_service}'.").format(
+                    room_id=room_jid_s,
+                    muc_service=unicode(muc_service))))
             if not room_jid.user:
                 room_jid.user, room_jid.host = room_jid.host, muc_service
         else: