Villager buyB tag doesn't work

When using /summon to spawn a villager.

I have the following command:

/summon Villager ~ ~1 ~ {Offers:{Recipes:[{buy:{id:388,Count:10},buyB:{id:1,Count:1},sell:{id:403,Count:1,tag:{StoredEnchantments:[{id:16,lvl:10}]}}}]}}

but the buyB tag doesn't work even though on the minecraftwiki, it is documented that you need to use buyB for the second 'cost' item.
Everyhing else seems to work, just not the second slot.