zfs kernel module on sparc64?

Is there some obvious trick that I'm missing for building the (-current)
zfs module on sparc64? 

Building a GENERIC kernel with `make buildkernel` does not build zfs.ko
on sparc64. Is there something in the kernel config that needs to be set
that I am perhaps foolishly overlooking? I've also tried building the
module directly in /usr/src/sys/modules/zfs/, but trying to kldload the
module just resulted in link_elf: symbol  undefined appearing on the
root console.

What have I missed?

Many thanks,
Jonathan.