busybox switch_root: not rootfs

I am following the TinyGentoo install from the gentoo
wiki.  I am able to install my tinygentoo on my host
machine without trouble (uclibc + busybox). 

When I try to switch my root initramfs to a new root
directory i have mounted by nfs i get an error.

My command is:
> switch_root /tmp/nfsroot /sbin/init

The error is:
switch_root: not rootfs 

Any ideas on how I might begin to debug this?
Thanks
Jon