diff --git a/federationapi/routing/send.go b/federationapi/routing/send.go
index 97425f52..40d4b033 100644
--- a/federationapi/routing/send.go
+++ b/federationapi/routing/send.go
@@ -620,7 +620,7 @@ func checkAllowedByState(e *gomatrixserverlib.Event, stateEvents []*gomatrixserv
 	return gomatrixserverlib.Allowed(e, &authUsingState)
 }
 
-var processEventWithMissingStateMutexes = &internal.MutexByRoom{}
+var processEventWithMissingStateMutexes = internal.NewMutexByRoom()
 
 func (t *txnReq) processEventWithMissingState(
 	ctx context.Context, e *gomatrixserverlib.Event, roomVersion gomatrixserverlib.RoomVersion,