This error comes up when enabling VotifierScripts.

http://dev.bukkit.org/paste/6971/

Plugin Version: 1.1-SNAPSHOT-b28
CraftBukkit Version: 1.4.7-R0.1 (Build #02602)

As far as I know, this bug only appears in the previous versions but will continue to appear in the latest version if you experienced it before upgrading. In this case, you must delete the scripts.bin file in the VotifierScripts plugin. This will cause any previously scheduled scripts not not be triggered however. Alternately, if you open that file in something like notepad++ you can remove the entry for the blank username.