--- a/taskcluster/taskgraph/actions/registry.py
+++ b/taskcluster/taskgraph/actions/registry.py
@@ -280,16 +280,18 @@ def trigger_action_callback(task_group_i
         create.testing = True
         taskcluster.testing = True
 
     # fetch the task, if taskId was given
     # FIXME: many actions don't need this, so move this fetch into the callbacks
     # that do need it
     if task_id:
         task = taskcluster.get_task_definition(task_id)
+    else:
+        task = None
 
     cb(Parameters(**parameters), graph_config, input, task_group_id, task_id, task)
 
 
 def _load(graph_config):
     # Load all modules from this folder, relying on the side-effects of register_
     # functions to populate the action registry.
     actions_dir = os.path.dirname(__file__)