Help with leveling non-hero ablilities

I created a spell based on the Crypt Lord's beetle creation spell, as a non-hero spell with 2 levels, causing different units to be spawned at each level. I also created an upgrade with the same properties as the Skeleton Mage upgrade for this spell, with a ability level bonus increment of 1 and a base of 1. However, when I use the upgrade in-game, the ability does not change! What is wrong?
And BTW, I can't just use the Skeleton Warrior spell because I want to have a spawn limit on the number of units.

I don't know much about leveling an unit ability with upgrades, untill you reminded me of summon skeletons I thought it couldn't be done. But anyway, you will be able to directly level an ability with triggers with the patch 1.17, so you can wait for that if you want.