diff --git a/force_bdss/core_driver_events.py b/force_bdss/core_driver_events.py
index b656bde49b803ea1f46f59aa0930422ba61c1270..2116c2a6e456515a80a38031bcdd65a71837124f 100644
--- a/force_bdss/core_driver_events.py
+++ b/force_bdss/core_driver_events.py
@@ -7,8 +7,8 @@ class BaseDriverEvent(HasStrictTraits):
 
 class MCOStartEvent(BaseDriverEvent):
     """MCO should emit this event when the evaluation starts."""
-    input_names = Str()
-    output_names = Str()
+    input_names = Tuple(Str())
+    output_names = Tuple(Str())
 
 
 class MCOFinishEvent(BaseDriverEvent):