freebsd stable - Nov 2004 - Is crash dumping supported onto a gmirror swap partition?

Is it possible to save crashdumps onto a gmirror device?

I'm trying to understand why one of my systems (being upgraded from
5.3BETA4 to 5.3) is sponteously rebooting.

The first step seems to be to try to get a crash dump, but:

   (merlin)[5:16pm]~>>sudo dumpon -v /dev/mirror/gm0s1b
   Password:
   dumpon: ioctl(DIOCSKERNELDUMP): Operation not supported
   (merlin)[5:16pm]~>>

Is the problem that one can't dump onto a gmirror'ed device, or is
there something else that I'm missing.

I'm running a default kernel, w/ an 'options smp' added.

g.