Baking GI and FG

Can anyone shed any light (no pun intended) on baking GI and FG

I have a simple scene set up with a dome, ground plane,cube and 1 light. when I bake GI it works perfectly but when I turn on FG the lightmaps have odd artifacts in them. the render of the the GI and FG before baking looks fine.
I would very much appreciate any tips.

Thanks