Modding, sprites & mindbLast

[sube]

I found this "funny" behaviour: I have a mod with some new units, each of them with a brand new sprite. These sprites seem OK in every situation: recruitment screen, unit view, army setup, battle, they look fine everywhere.

BUT, when these units are paralyzed (I see it usually when they are hit by the Illithid mind bLast), the black background that you have in the sprite shows up in the battle screen, it is not seen as transparent anymore.

Any explanation/suggestion on how to solve that?

Thanks!