diff --git a/tools_webrtc/flags_compatibility.py b/tools_webrtc/flags_compatibility.py
index 8265537..d3570a5 100644
--- a/tools_webrtc/flags_compatibility.py
+++ b/tools_webrtc/flags_compatibility.py
@@ -21,7 +21,7 @@
 
   test_command = _ForcePythonInterpreter(unrecognized_args)
   if args.isolated_script_test_perf_output:
-    test_command += ['--isolated_script_test_perf_output',
+    test_command += ['--isolated_script_test_perf_output=' +
                      args.isolated_script_test_perf_output]
   logging.info('Running %r', test_command)