diff --git a/css_browserhook.py b/css_browserhook.py
index eb4615d..5b48f64 100644
--- a/css_browserhook.py
+++ b/css_browserhook.py
@@ -47,7 +47,7 @@ async def evaluate_js(self, js, run_async=False, get_result=True):
                 if "value" in res["result"]["result"]:
                     res = res["result"]["result"]["value"]
                 elif "type" in res["result"]["result"] and res["result"]["result"]["type"] == "undefined":
-                    res = "Undefined"
+                    res = None
                 else:
                     res = None
             else: