Trouble building w3-url-e21

[csj <csj@mindgate.net>]

I'm having trouble building w3-url-e21 (2001.11.08). The relevant
lines of stderr appears to be:

NLoading cus-dep...
Directory .
Generating cus-load.el...
Wrong type argument: stringp, (fancy-splash-image custom-variable)
make[2]: *** [custom-load.el] Error 255
make[1]: *** [url] Error 2
make: *** [build-stamp] Error 2
debuild: fatal error at line 456:
dpkg-buildpackage failed!

The problem appears to be the building of custom-load.el. In
lisp/Makefile, I see the following:

custom-load.el: $(SOURCES) $(URLSOURCES)
        $(EMACS_CMD) -f emacs-batch-build-custom-load $(srcdir)
        if [ -f cus-load.el ]; then mv cus-load.el custom-load.el; fi

Is this a bug? Or just a problem with my particular setup? Not
having the time (nor fortitude) to pore over the lisp sources, I
managed to finish the build by manually copying over cus-load.el
to custom-load.el.