mercurial/commands.py

--- a/mercurial/commands.py	Sat Mar 16 21:36:44 2013 -0500
+++ b/mercurial/commands.py	Sat Mar 16 22:48:22 2013 -0500
@@ -869,6 +869,8 @@
         marks[mark] = tgt
         if not inactive and cur == marks[mark]:
             bookmarks.setcurrent(repo, mark)
+        elif cur != tgt and mark == repo._bookmarkcurrent:
+            bookmarks.setcurrent(repo, None)
         marks.write()
 
     # Same message whether trying to deactivate the current bookmark (-i