diff --git a/testing/web-platform/mach_commands.py b/testing/web-platform/mach_commands.py
index ed3a766d1402..9e43ed6f8fed 100644
--- a/testing/web-platform/mach_commands.py
+++ b/testing/web-platform/mach_commands.py
@@ -46,8 +46,7 @@ class WebPlatformTestsRunnerSetup(MozbuildObject):
             kwargs["certutil_binary"] = self.get_binary_path('certutil')
 
         if kwargs["stackfix_dir"] is None:
-            kwargs["stackfix_dir"] = os.path.split(
-                self.get_binary_path(validate_exists=False))[0]
+            kwargs["stackfix_dir"] = self.bindir
 
         here = os.path.split(__file__)[0]