diff --git a/synapse/event_auth.py b/synapse/event_auth.py
index dba1f6841d..0b2c741398 100644
--- a/synapse/event_auth.py
+++ b/synapse/event_auth.py
@@ -828,8 +828,8 @@ def _can_send_event(event: "EventBase", auth_events: StateMap["EventBase"]) -> b
     # Check state_key
     if hasattr(event, "state_key"):
         if event.state_key.startswith("@"):
-            if event.state_key != event.user_id and (
-                not use_msc3779 or not event.state_key.startswith(event.user_id + "_")
+            if event.state_key != event.user_id and not (
+                use_msc3779 and event.state_key.startswith(event.user_id + "_")
             ):
                 raise AuthError(403, "You are not allowed to set others state")