--- a/python/mozbuild/mozbuild/test/action/test_generate_browsersearch.py
+++ b/python/mozbuild/mozbuild/test/action/test_generate_browsersearch.py
@@ -27,17 +27,17 @@ test_data_path = mozpath.join(test_data_
 
 class TestGenerateBrowserSearch(unittest.TestCase):
     """
     Unit tests for generate_browsersearch.py.
     """
 
     def _test_one(self, name):
         with TemporaryDirectory() as tmpdir:
-            with NamedTemporaryFile(mode='rw') as temp:
+            with NamedTemporaryFile(mode='r+') as temp:
                 srcdir = os.path.join(test_data_path, name)
 
                 generate_browsersearch.main([
                     '--verbose',
                     '--srcdir', srcdir,
                     temp.name])
                 return json.load(temp)