2 files changed, 7 insertions, 5 deletions
diff --git a/CHANGES.rst b/CHANGES.rst
index 68e9d8c671..9106134b46 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -30,6 +30,7 @@ Changes in synapse v0.18.6 (2017-01-06)
 Bug fixes:
 
 * Fix bug when checking if a guest user is allowed to join a room (PR #1772)
+  Thanks to Patrik Oldsberg for diagnosing and the fix!
 
 
 Changes in synapse v0.18.6-rc3 (2017-01-05)
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index 8a76469b77..b2806555cf 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -232,11 +232,12 @@ class RoomMemberHandler(BaseHandler):
                     errcode=Codes.BAD_STATE
                 )
 
-            same_content = content == old_state.content
-            same_membership = old_membership == effective_membership_state
-            same_sender = requester.user.to_string() == old_state.sender
-            if same_sender and same_membership and same_content:
-                defer.returnValue(old_state)
+            if old_state:
+                same_content = content == old_state.content
+                same_membership = old_membership == effective_membership_state
+                same_sender = requester.user.to_string() == old_state.sender
+                if same_sender and same_membership and same_content:
+                    defer.returnValue(old_state)
 
         is_host_in_room = yield self._is_host_in_room(current_state_ids)