diff --git a/synapse/handlers/auth.py b/synapse/handlers/auth.py
index 2b0c50513095..2cf2a4665eb5 100644
--- a/synapse/handlers/auth.py
+++ b/synapse/handlers/auth.py
@@ -481,8 +481,9 @@ async def check_ui_auth(
 
         sid: Optional[str] = None
         authdict = clientdict.pop("auth", {})
-        if "session" in authdict:
-            sid = authdict["session"]
+        if authdict is not None:
+            if "session" in authdict:
+                sid = authdict["session"]
 
         # Convert the URI and method to strings.
         uri = request.uri.decode("utf-8")