hi,
sounds like you've copied the ECOFF-format kernel
onto the disk, or (perhaps) you haven't installed bootblocks.

	* The DECstation net-boot PROM only knows
	  how to boot ECOFF format binaries.
	  (That's the format Ultrix uses).

	* The native NetBSD  bootblocks only know
	  how to boot a.out format binaries, (since
	  that's what NetBSD/pmax 0.9 or thereabouts used.)

Are you sure you have the non-ecoff kernel on your disk?

If so, have you  done

	disklabel -B <disk>

to install bootblocks onto the disk you're trying to boot?