diff -r 5ed6802e6bcb -r 18c47562d331 hgext/bookmarks.py
--- a/hgext/bookmarks.py	Thu Jul 22 21:43:45 2010 +0200
+++ b/hgext/bookmarks.py	Sat Jul 24 11:20:21 2010 +0300
@@ -136,6 +136,9 @@
         if "\n" in mark:
             raise util.Abort(_("bookmark name cannot contain newlines"))
         mark = mark.strip()
+        if not mark:
+            raise util.Abort(_("bookmark names cannot consist entirely of "
+                               "whitespace"))
         if mark in marks and not force:
             raise util.Abort(_("a bookmark of the same name already exists"))
         if ((mark in repo.branchtags() or mark == repo.dirstate.branch())