diff -r 9cb1f82a2571 -r e0f06228bb66 mercurial/archival.py
--- a/mercurial/archival.py	Wed Feb 25 11:39:14 2015 -0800
+++ b/mercurial/archival.py	Sun Feb 15 17:21:48 2015 -0500
@@ -230,7 +230,7 @@
     }
 
 def archive(repo, dest, node, kind, decode=True, matchfn=None,
-            prefix=None, mtime=None, subrepos=False):
+            prefix='', mtime=None, subrepos=False):
     '''create archive of repo as it was at node.
 
     dest can be name of directory, name of archive file, or file