NIL is used in many different contexts and for many different purposes in Lisp.
The error message makes me suspect that this is an entirely new issue.

I will need more debugging information in order to diagnose this one.
A debugger stack backtrace would be very useful.
Along with some more detailed informations about your setup, like
how was your CCL installed (stand alone or as part of Lispbox),
which version of CCL is this (ie: the value returned by (lisp-implementation-version))...

Cheers,

Jean-Claude