edit form base not working

Hello all,

I have just started working with V8i (8.11.05.46) and am having problems with this command.

I have drawn a slab form, using the place slab form tool. Now I want to change it, using edit form base. This gives me the message: error:the base shape self intersects. Or: Unable to create form. Using the same tool on a free form works OK. Is this a known problem, or am I missing something? I am very experienced with V8-2004, so I don't think the obvious would slip my mind.

TIA, André