Widescreen Matte asking as Mask

If I have two video tracks and apply a widescreen mask to the top mask (to make 4:3 look 16:9) the matte inevitably works as a mask, and shows the underlying track -- I just want it to be black. I don't want it to expose anything.

Is there something that I accidentally toggled on to make this happen, and how do I turn it off?

I don't want to have to clean-up the tracks underneath the track I want to show. (And adding a matte to them doesn't always work because some of them... well, it's a long explanation, but it doesn't.)

Mike