crash when trading with a villager in a custom map

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DESCRIPTION:
Minecraft would crash every time I tried to check the trade interface of a custom villager

WHAT I EXPECTED TO HAPPEN:
The trade interface would open and then I could trade/ check the items

WHAT ACTUALLY HAPPENED:
The regular dirt-background came up with no text and was then replaced by a crash report in the minecraft launcher

STEPS TO REPRODUCE:
1. Create a custom villager with items in the 'buy' and 'sell' values in a command block
2. Trigger the command block for villager to spawn
3. Right click to trade (as you would normally do)
4. Minecraft crashes

Crash report Minecraft version 1.7.10
Mods: none

This is MC-36324