cannot NFS export a unionfs

[Coert]

Hello all!

My server has three disks, and for ease of backup also 3 seperate filesystems.
In the past, (when it still ran Linux,) I union mounted those 3 filesystems on 
a seperate mount point, and exported that via NFS.

I am trying to do the same now on FreeBSD 10.0, and the union mount works on 
the local machine, but if I try to NFS export it, I get an error:
unionfs does not support mount update. 
(something along those lines, sorry I am not in front of the box right now)

I have googled all over the place, so far with no luck.

Could anyone give me some pointers?
Should I use FUSE union instead?
Should I use something other than unionfs?


Kind regards,
Coert