Cantrip Scaling

I'm trying to create my own cantrip based on Booming Blade using the toolkit. I've got pretty much everything down, except when I hit level 5 casting the spell crashes the game-

I know in my head this is because the game doesn't know how to scale the spell. I've looked through all the spell lists in vanilla and only found the one entry for Booming Blade in GustavDev. I've looked through the stats for it and can't find anything relevant that I haven't already copied over. There's several tutorials for spells in mod.io, but upcasting seems to be handled completely differently from cantrip scaling.

Preliminary google searches have turned up nothing specifically addressing this.

What am I missing?