After feeding two tamed llamas several hay bales while Strength is 0 (which is the default when summoned), the moment they began to breed, the game crashes.

Crashes when bred:

/summon minecraft:llama ~ ~1 ~ {Tame:1b,Strength:0}

Does not crash:

/summon minecraft:llama ~ ~1 ~ {Tame:1b,Strength:1}