[Exim] $message_headers not recognised [Exim 4.04]

We have a router condition in our configuration file that looks like this:

condition = ${perl{check_content}{$message_headers}{$message_body}}

Every few messages, we get this error message:

2002-05-20 16:20:22 failed to expand condition"${perl{check_content}{$message_headers} {$message_body}}" for content_checker router: unknown variable name "message_headers"

It doesn't happen every message (almost all messages pass through this router)
and no message seems to be adversely affected in its delivery by this failure.

Any ideas?
___________________________________________________________________________
Ephraim Silverberg, CSE System Group,        Phone number:    972-2-6585521
Hebrew University, Jerusalem, Israel.        Fax number:      972-2-5617723
WWW: http://www.cs.huji.ac.il/~ephraim       E-mail: ephraim@???