diff --git a/src/brad/front_end/front_end.py b/src/brad/front_end/front_end.py
index 178ea9e6..f460e2bf 100644
--- a/src/brad/front_end/front_end.py
+++ b/src/brad/front_end/front_end.py
@@ -199,6 +199,7 @@ def __init__(
 
     def _handle_query_from_flight_sql(self, query: str) -> RowList:
         assert self._flight_sql_server_session_id is not None
+        assert self._main_thread_loop is not None
 
         future = asyncio.run_coroutine_threadsafe(
             self._run_query_impl(self._flight_sql_server_session_id, query, {}),