diff --git a/phabricator/conduit.py b/phabricator/conduit.py
--- a/phabricator/conduit.py
+++ b/phabricator/conduit.py
@@ -81,10 +81,15 @@
         url = self._url + method
         try:
             response = urlgrabber.urlopen(url, headers=headers, data=req_data)
-            response = json.load(response)
         except URLGrabError as ex:
             raise Client(ex.errno, str(ex))
 
+        try:
+            response = json.load(response)
+        except ValueError:
+            # Can't decode the data, not valid JSON (html error page perhaps?)
+            raise ClientError(-1, 'did not receive a valid JSON response')
+
         if response['error_code'] is not None:
             raise ClientError(response['error_code'], response['error_info'])
         return response['result']