--- a/testing/mozharness/mozharness/base/config.py
+++ b/testing/mozharness/mozharness/base/config.py
@@ -160,17 +160,17 @@ def parse_config_file(file_name, quiet=F
         config = local_dict[config_dict_name]
     elif file_name.endswith('.json'):
         fh = open(file_path)
         config = {}
         json_config = json.load(fh)
         config = dict(json_config)
         fh.close()
     else:
-        raise RuntimeError("Unknown config file type %s!" % file_name)
+        raise RuntimeError("Unknown config file type %s! (config files must end in .json or .py)" % file_name)
     # TODO return file_path
     return config
 
 
 def download_config_file(url, file_name):
     n = 0
     attempts = 5
     sleeptime = 60