Installboot uses wrong device for secondary boot loader

I'm installing 6.3 on a RAID1. Install was fine until ending with an error message, "invalid boot record signature…".

I manually ran installboot:
>. installboot -v -r /mnt sd4

Hand transcription:

Using /mnt as root
Installing bootstrap on /dev/rsd4c
Using first-stage /mnt/usr/mdec/biosboot, second-stage /mnt/usr/mdec/boot
sd4: softraid volume with 2 disk(s)
sd4: installing boot loader on softraid volume
/mnt/usr/mdec/boot is 6 blocks x 16384 bytes
sd0a: installing boot blocks on /dev/rsd0c, part offset 144
Master boot record (MBR) at sector 0
Install boot: invalid boot record signature (0x0000) @ sector 0

I did not typo the secondary boot block install. It attempts to install on sd0 instead of sd4 as specified in my command.

Eric