--- a/testing/web-platform/tests/tools/webdriver/webdriver/transport.py
+++ b/testing/web-platform/tests/tools/webdriver/webdriver/transport.py
@@ -58,17 +58,17 @@ class HTTPWireProtocol(object):
 
         self.host = host
         self.port = port
         self.url_prefix = url_prefix
 
         self._timeout = timeout
 
     def url(self, suffix):
-        return urlparse.urljoin(self.path_prefix, suffix)
+        return urlparse.urljoin(self.url_prefix, suffix)
 
     def send(self, method, url, body=None, headers=None):
         """Send a command to the remote.
 
         :param method: "POST" or "GET".
         :param url: "command part" of the requests URL path
         :param body: Body of the request.  Defaults to an empty dictionary
             if ``method`` is "POST".