diff -r c0a6c19690ff -r 75979c8d4572 mercurial/debugcommands.py
--- a/mercurial/debugcommands.py	Thu Oct 12 19:20:04 2017 -0700
+++ b/mercurial/debugcommands.py	Thu Oct 12 23:30:46 2017 -0700
@@ -2072,7 +2072,7 @@
     If the update succeeds, retry the original operation.  Otherwise, the cause
     of the SSL error is likely another issue.
     '''
-    if pycompat.osname != 'nt':
+    if not pycompat.iswindows:
         raise error.Abort(_('certificate chain building is only possible on '
                             'Windows'))