Key framing crop effect on a title?

Hi,

I have a title on my timeline and wanted to do a keyframed crop so the title disappears letter by letter. I've very little experience with titles but I though I'd just apply 3D Warp and key frame the crop however when I apply the effect it no longer becomes a transparent title so the video underneath disappears.

I obviously approaching this the wrong way but would appreciate an explanation as to what is going on here or an alternative way of achieving this?

Thanks