[Exim] weird log entry

I have Exim 3.02 installed on Debian, and yesterday I started finding this
entry in my panic log:

1999-07-26 16:50:40 failed to open when attempting to check NULL: No such
file or directory
1999-07-27 01:08:50 failed to open when attempting to check NULL: No such
file or directory

What could this mean? I looked through my config and "NULL" does not seem
to be mentioned in it, nor in my filters.

thanks

--
Andrew