diff --git a/synapse/api/auth.py b/synapse/api/auth.py
index f7cf17e433..75b7c467b5 100644
--- a/synapse/api/auth.py
+++ b/synapse/api/auth.py
@@ -20,7 +20,7 @@ from twisted.internet import defer
 from synapse.api.constants import EventTypes, Membership, JoinRules
 from synapse.api.errors import AuthError, Codes, SynapseError
 from synapse.util.logutils import log_function
-from synapse.types import EventID, RoomID, UserID
+from synapse.types import RoomID, UserID
 
 import logging
 
@@ -66,10 +66,10 @@ class Auth(object):
                 return True
 
             creating_domain = RoomID.from_string(event.room_id).domain
-            originating_domain = EventID.from_string(event.event_id).domain
+            originating_domain = UserID.from_string(event.sender).domain
             if creating_domain != originating_domain:
                 if not self.can_federate(event, auth_events):
-                    raise SynapseError(
+                    raise AuthError(
                         403,
                         "This room has been marked as unfederatable."
                     )
diff --git a/tests/test_state.py b/tests/test_state.py
index 5845358754..04c4439183 100644
--- a/tests/test_state.py
+++ b/tests/test_state.py
@@ -35,7 +35,7 @@ def create_event(name=None, type=None, state_key=None, depth=2, event_id=None,
 
     if not event_id:
         _next_event_id += 1
-        event_id = str(_next_event_id)
+        event_id = "$%s:test" % (_next_event_id,)
 
     if not name:
         if state_key is not None: