The error message like this:

,--------
| Signaling: (wrong-type-argument stringp nil)
|   insert-file-contents(nil)
|   gnus-agent-expire()
| * call-interactively(gnus-agent-expire)
|   execute-extended-command(nil)
| * call-interactively(execute-extended-command)
`--------

Here is the patch.

1999-07-06  Shenghuo ZHU  <zsh@cs.rochester.edu>

	* gnus-util.el (gnus-write-active-file): Use real name.

	* gnus-agent.el (gnus-agent-expire): Update active file 
	method by method.