Crash on trade with Villager.

Perform this command via a command block:

/summon Villager ~1 ~ ~ {Profession:2,Career:1,Offers:{Recipes:[{buy:{id:emerald,Count:1},sell:{id:air,Count:1}}]}}

This will produce a trade screen like this, with seemingly nothing in the selling area.

Now, attempt to trade with him. Upon placing an emerald in the trading slot, your game will crash. A crash log is attached.