Obtaining a kernel dump is not possible

Hello,

my ThinkPad gets a kernel panic if I boot a kernel with atapicam 
enabled. So I try to obtain a crash dump to get a backtrace. 
Unfortunately it seems so, that the kernel crashs quit early. Although I 
put dumpdev=/dev/ad0s1b in my loader.conf, I can't call doadump without 
getting the message: "Partition too small. Dump failed."

I think, that the dumpdev hasn't been assigned at that specific moment 
to the dumperinfo. Or is there another way to specify the dumpdev?

How can I all the same obtain a crash dump?

Regards,

Oliver Fischer