diff --git a/smach/src/smach/concurrence.py b/smach/src/smach/concurrence.py
index e814c0c..621dd48 100644
--- a/smach/src/smach/concurrence.py
+++ b/smach/src/smach/concurrence.py
@@ -193,7 +193,7 @@ def add(label, state, remapping={}):
         return state
 
     ### State interface
-    def execute(self, parent_ud = smach.UserData()):
+    def execute(self, parent_ud = None):
         """Overridden execute method.
         This starts all the threads.
         """
diff --git a/smach/src/smach/iterator.py b/smach/src/smach/iterator.py
index 57b1a87..50eb93e 100644
--- a/smach/src/smach/iterator.py
+++ b/smach/src/smach/iterator.py
@@ -135,7 +135,7 @@ def set_contained_state(
         self._final_outcome_map = final_outcome_map
 
     ### State interface
-    def execute(self, parent_ud):
+    def execute(self, parent_ud = None):
         self._is_running = True
 
         # Copy input keys
diff --git a/smach/src/smach/state_machine.py b/smach/src/smach/state_machine.py
index c6e0ed5..6223d39 100644
--- a/smach/src/smach/state_machine.py
+++ b/smach/src/smach/state_machine.py
@@ -321,7 +321,7 @@ def _update_once(self):
         return None
 
     ### State Interface
-    def execute(self, parent_ud = smach.UserData()):
+    def execute(self, parent_ud = None):
         """Run the state machine on entry to this state.
         This will set the "closed" flag and spin up the execute thread. Once
         this flag has been set, it will prevent more states from being added to