Invisible Collision DIF Casting Shadows

Does anyone know of a way to create an invisible dif that does not cast shadows? I've tried assigning a null texture to it but it still casts shadows. What I am trying to do is create a dif staircase but I'd like the actual collision geometry of the stairs to be a ramp (so that the character doesn't "pop" up every stair).

I have tried using a physicalized zone but I do not wish to have a force applied onto my player, I only wish to have a simple collision box that is invisible and does not cast shadows onto other objects. I have also tried using a DTS collision mesh but it doesn't work very well because it cannot receive player shadows and therefore it looks like the player is floating.

Thanks in advance for any suggestions.