diff --git a/buildtrigger/triggerutil.py b/buildtrigger/triggerutil.py
index b60f38620..a3d82cedc 100644
--- a/buildtrigger/triggerutil.py
+++ b/buildtrigger/triggerutil.py
@@ -52,7 +52,7 @@ def determine_build_ref(run_parameters, get_branch_sha, get_tag_sha, default_bra
     value = run_parameters['branch_name']
 
   kind = kind or 'branch'
-  value = value or default_branch
+  value = value or default_branch or 'master'
 
   ref = 'refs/tags/' + value if kind == 'tag' else 'refs/heads/' + value
   commit_sha = get_tag_sha(value) if kind == 'tag' else get_branch_sha(value)