Villager loot tables

[Tskuno]

For some reason I have a villager who sells air, you give an emerald and he won't accept the trade as there's just. Nothing there. Using the wrech tool(forget it's name) I can find the offers he gives, and it lists it as Minecraft:air. I'm unsure why he's offering that as it just clogs up his offers

[Tskuno]

This is 1.12.2.