MDD export error ... Help!

Hi Guys,

Trying to export a small animation from Blender as MDD. When I File, export with MDD I get this:

“Number of verts has changed during animation, cannot export”

Any ideas why this is so?

I have the mesh Subsurfaced - I notice when I dump or turn off the Subsurface Modifier on the Mesh this error does not occur. I may have answered my own question here, but am interested in what the error means. I animated it with SubSurf on.