Milwaukee M12/M18 charger flashing red/green

Just got a brand new cordless drill/impact driver set (link) and I'm trying to give the batteries their first charge. For some reason M18 batteries cause the charger to just blink red/green except when I put the charger at 90º vertical. Laying flat doesn't work. Anyone know what the deal is?

*edit, got it to work. See my comment below. Leaving the post up just in case future dumbasses similar to myself search the internet in hopes of a solution.

*double edit 8 months later since the comments are collapsed by default:

I'm just a dumbass and wasn't pushing the battery in far enough to seat the prongs correctly. I swear I tried pushing it down harder earlier and it didn't feel like it was going to budge.