--- a/taskcluster/taskgraph/actions/registry.py
+++ b/taskcluster/taskgraph/actions/registry.py
@@ -300,17 +300,18 @@ def render_actions_json(parameters):
 def trigger_action_callback(task_group_id, task_id, task, input, callback, parameters,
                             test=False):
     """
     Trigger action callback with the given inputs. If `test` is true, then run
     the action callback in testing mode, without actually creating tasks.
     """
     cb = get_callbacks().get(callback, None)
     if not cb:
-        raise Exception('Unknown callback: {}'.format(callback))
+        raise Exception('Unknown callback: {}. Known callbacks: {}'.format(
+            callback, get_callbacks().keys()))
 
     if test:
         util.testing = True
 
     cb(Parameters(**parameters), input, task_group_id, task_id, task)
 
 
 @memoize