diff --git a/libcxx/utils/libcxx/test/config.py b/libcxx/utils/libcxx/test/config.py
index 9d4f7f33747e..df25d949c096 100644
--- a/libcxx/utils/libcxx/test/config.py
+++ b/libcxx/utils/libcxx/test/config.py
@@ -280,7 +280,7 @@ class Configuration(object):
         elif self.use_system_cxx_lib == 'false':
             self.use_system_cxx_lib = False
         else:
-            assert os.path.isdir(self.use_system_cxx_lib)
+            assert os.path.isdir(str(self.use_system_cxx_lib))
         self.lit_config.note(
             "inferred use_system_cxx_lib as: %r" % self.use_system_cxx_lib)