diff --git a/service/history/workflow/mutable_state_impl.go b/service/history/workflow/mutable_state_impl.go
index 0438386767e..6b96eac4206 100644
--- a/service/history/workflow/mutable_state_impl.go
+++ b/service/history/workflow/mutable_state_impl.go
@@ -1048,6 +1048,7 @@ func (e *MutableStateImpl) DeleteActivity(
 	}
 
 	delete(e.updateActivityInfos, scheduleEventID)
+	delete(e.syncActivityTasks, scheduleEventID)
 	e.deleteActivityInfos[scheduleEventID] = struct{}{}
 	return nil
 }