diff --git a/mercurial/url.py b/mercurial/url.py
--- a/mercurial/url.py
+++ b/mercurial/url.py
@@ -498,7 +498,11 @@ def _verifycert(cert, hostname):
     for s in cert.get('subject', []):
         key, value = s[0]
         if key == 'commonName':
-            certname = value.lower()
+            try:
+                # 'subject' entries are unicode
+                certname = value.lower().encode('ascii')
+            except UnicodeEncodeError:
+                return _('IDN in certificate not supported')
             if (certname == dnsname or
                 '.' in dnsname and certname == '*.' + dnsname.split('.', 1)[1]):
                 return None
diff --git a/tests/test-url.py b/tests/test-url.py
--- a/tests/test-url.py
+++ b/tests/test-url.py
@@ -36,3 +36,7 @@ check(_verifycert({'subject': ()},
       'no commonName found in certificate')
 check(_verifycert(None, 'example.com'),
       'no certificate received')
+
+# Unicode (IDN) certname isn't supported
+check(_verifycert(cert(u'\u4f8b.jp'), 'example.jp'),
+      'IDN in certificate not supported')