custom package error: dpkg -P tries to remove /opt

[Mike Fogel <mike@opendns.com>]

hello all debian-devel,

This seems like a very basic issue, but I've searched through the 
mailing lists and web and haven't found any help.

I'm trying to create a basic set of custom packages which will hold some 
of our software, complied and customized as we like it for our machines. 
 We like to put our custom build software in /opt.  Everything is 
working fine, I have basic postinst and prerm scripts working well.

However, I can't seem to figure out how to resolve this error:

$ dpkg -i custom-package.deb
 ... installation goes perfectly ....
$ dpkg -P custom-package
 ... removal goes perfectly until this error/warning....
dpkg - warning: while removing custom-package, unable to remove 
directory `/opt': Device or resource busy - directory may be a mount point ?

Well, yeah, /opt is a mount point, and it would definitely be a good 
thing if it wasn't removed.  But how can I tell dpkg that I don't want 
it to try to remove /opt??

Thanks much!!!

Mike