--- a/taskcluster/taskgraph/cron/util.py
+++ b/taskcluster/taskgraph/cron/util.py
@@ -21,9 +21,12 @@ def match_utc(params, hour=None, minute=
     if hour and params['time'].hour != hour:
         return False
     if minute and params['time'].minute != minute:
         return False
     return True
 
 
 def calculate_head_rev(options):
-    return subprocess.check_output(['hg', 'log', '-r', options['head_ref'], '-T', '{node}'])
+    # we assume that run-task has correctly checked out the revision indicated by
+    # GECKO_HEAD_REF, so all that remains is to see what the current revision is.
+    # Mercurial refers to that as `.`.
+    return subprocess.check_output(['hg', 'log', '-r', '.', '-T', '{node}'])