occasional failure to rename

[knubee]

Usually I am able to rename files in dired, but sometimes emacs throws
an error when I try to rename/move a file or directory:

  dired-rename-file: Symbol's value as variable is void: dired-backup-
overwrite

This does not happen all the time, but when it does the only solution
I have found is to close out of emacs and restart.

A web/newsgroup search does not turn up anything. Anyone know a) what
could be causing this, and b) a less laborious way to resolve it if/
when it does occur?