Mask in a PreComp less than 100% scaled

Hi guys.

I'm trying to apply a mask to a precomp that is previously animated in scale. Seems that AE doesn't like this as I cannot even put a mask point different than the first one. The only way to apply a mask is if the precomp is set to 100% scale but that doesn't fit my needs as I want the mask size to be set when the precomp is at a certain scale.

To picture the scenario:

I have the precomp scale animated from 0% to 23% and I want to apply a mask using this precomp size (23%) as reference. Later I will modify the precomp settings ("canvas" to understand better) to make it larger to fit the empty space of a matte in the "mother" comp and will also add another scale keyframe to make it bigger but the mask will be already set up to retain proportions. As I said, when I try to put mask points when precomp is at 23% it is impossible to put more than one.

Any ideas to solve this?

Thanks!