--- a/mercurial/hgweb/server.py	Tue Jun 12 20:42:42 2018 -0400
+++ b/mercurial/hgweb/server.py	Tue Jun 12 20:43:56 2018 -0400
@@ -215,6 +215,7 @@
         self.sent_headers = True
 
     def _start_response(self, http_status, headers, exc_info=None):
+        assert isinstance(http_status, str)
         code, msg = http_status.split(None, 1)
         code = int(code)
         self.saved_status = http_status