Light wrap - no source layer available?!

Hi folks,
Probably a... newbie problem - but since I am not shy I'll ask anyway. :)
I have a short movie sequence consisting of two layers:
1) Foreground object (PNG sequence - works perfect)
2) Bright background image
Now I'd like to apply the light wrap effect.
Ok, I apply the effect to the foreground layer (a composite shot).
But when I try to choose the background layer as "source layer" there is no layer available in the menu.
Probably a simple problem... something with wrong layer properties or so...?
Thanks in advance!