Help with upgrading a spell!

OK... I'm trying to make an upgradable spell, like the Raise Dead spell, so that at base level it spawns a certain unit, and when an upgrade is researched a different unit is spawned. I've made a custom 2-level non-hero spell, and an upgrade that increases the ability level with:
ability level bonus- increment =1
ability level bonus-base =1

(Those are the same settings as the Skeletal Mastery upgrade)

However, so far I cannot get the upgrade to work ingame. When I reasearch the skill, nothing happens. Whats wrong?

make sure the unit is allowed to use the upgrade. There is a field called "techtree - Upgrades used". put your upgrade there.