diff --git a/cylc/flow/simulation.py b/cylc/flow/simulation.py
index 3b737eec642..c8d91c93356 100644
--- a/cylc/flow/simulation.py
+++ b/cylc/flow/simulation.py
@@ -75,7 +75,7 @@ def __init__(
         self,
         itask: 'TaskProxy',
         db_mgr: 'WorkflowDatabaseManager',
-        rtconfig
+        rtconfig: Dict[str, Any]
     ):
 
         # itask.summary['started_time'] and mode_settings.timeout need