diff -r b2316265a32e -r d1fccbd50fcd mercurial/hgweb/hgweb_mod.py
--- a/mercurial/hgweb/hgweb_mod.py	Thu Oct 05 14:48:31 2017 -0400
+++ b/mercurial/hgweb/hgweb_mod.py	Thu Oct 05 14:48:52 2017 -0400
@@ -187,7 +187,7 @@
         if style == styles[0]:
             vars['style'] = style
 
-        start = '&' if req.url[-1] == '?' else '?'
+        start = r'&' if req.url[-1] == r'?' else r'?'
         sessionvars = webutil.sessionvars(vars, start)
 
         if not self.reponame: