hmmm strange...
try visualising something, start small (like imagine an apple in front of you or something) then build up a more and more elaborate dream world around you. Thats sounds cool anyway, its like you get a blank dream canvas.

Ive experienced something a bit similar. Sometimes I get to a stage where I keep slipping in and out of wake and dreaming, in and out in and out so that I can neve rhang onto a dream, then if I try to visualise something (usually sex ) I just get whatever I am thinking about in an empty void...a bit of a 2 dimentional dream.