--- a/mercurial/posix.py	Mon Feb 02 14:26:47 2015 -0500
+++ b/mercurial/posix.py	Wed Feb 11 19:57:07 2015 +0900
@@ -322,7 +322,7 @@
     global _needsshellquote
     if _needsshellquote is None:
         _needsshellquote = re.compile(r'[^a-zA-Z0-9._/-]').search
-    if not _needsshellquote(s):
+    if s and not _needsshellquote(s):
         # "s" shouldn't have to be quoted
         return s
     else: