--- a/testing/marionette/mach_commands.py
+++ b/testing/marionette/mach_commands.py
@@ -39,22 +39,22 @@ def run_marionette(tests, b2g_path=None,
     from marionette.runtests import (
         MarionetteTestRunner,
         BaseMarionetteArguments,
         MarionetteHarness
     )
 
     parser = BaseMarionetteArguments()
     commandline.add_logging_group(parser)
-    args = parser.parse_args()
 
     if not tests:
         tests = [os.path.join(topsrcdir,
                  'testing/marionette/client/marionette/tests/unit-tests.ini')]
-    args.tests = tests
+
+    args = parser.parse_args(args=tests)
 
     if b2g_path:
         args.homedir = b2g_path
         if emulator:
             args.emulator = emulator
     else:
         args.binary = binary
         path, exe = os.path.split(args.binary)