I think I have a very dumb question, but in all the books I have it does not specify if blend shapes works on all surfaces, i.e. nurbs, polys, subs, - does it?

I have 6 poly heads all tweaked into vowels, and when I try to create a blend shape, it says "no deformable objects selected"? I did it with nurbs before, but don't understand why it won't work? I converted to subs and it said the same thing.
I deleted the history, and nothing.

any ideas please?

I am choosing the shaped heads, and then the main head last.

also, where in the deformation list do I place the blendshape when applying it to a smoothly bound character? I read you can do it, but the deformation order is important. Does it go before, or after clusters and tweaks?