How to keep freebsd-update from trashing custom kernel?

[Brett Glass]

Everyone:

Just ran freebsd-update (fetch, then install) on a system on which 
I run a customized kernel, and discovered that it has overwritten 
my custom kernel... even though I'd copied the original to 
/boot/GENERIC when I first installed the system. I was under the 
impression that creating /boot/GENERIC, and putting the GENERIC 
kernel in it, would cause freebsd-update to update that directory 
rather than one's custom kernel. I now must rebuild the kernel to 
keep the machine working.

What went wrong, and how do stop it from recurring?

--Brett Glass