diff --git a/mercurial/dirstate.py b/mercurial/dirstate.py
--- a/mercurial/dirstate.py
+++ b/mercurial/dirstate.py
@@ -38,7 +38,12 @@ class dirstate(object):
         self._opener = opener
         self._validate = validate
         self._root = root
-        self._rootdir = os.path.join(root, '')
+        # ntpath.join(root, '') of Python 2.7.9 does not add sep if root is
+        # UNC path pointing to root share (issue4557)
+        if root.endswith(os.sep):
+            self._rootdir = root
+        else:
+            self._rootdir = root + os.sep
         self._dirty = False
         self._dirtypl = False
         self._lastnormaltime = 0