Getting custom-type global from Lua?

[L-28C <kixdemp@...>]

Hello people of Lua town... or list, same thing. :-)

I'm using this function to retrieve Lua global 'BottomLayer' of type 
'Image'. I try it but it returns a null pointer:

Image *Map::GetBottomLayer()
{
	lua_getglobal(Script, "BottomLayer");
	return (Image*)tolua_tousertype(Script, 1, 0);
}

I just grabbed the tolua_tousertype from the toLua binding of the Image 
class. I Googled a bit and the arguments seem alright... What am I doing 
wrong?

Thanks in advance!