Crash upon loading world with custom spawners

One of my worlds crashed upon loading.

I was playing around with custom mob spawners to create a skeleton with an iron sword, an iron helmet, a chain chestplate, and dyed leather leggings and boots. The skeleton would have had resistance three, jump boost one, and strength two potion effects applied to it as part of it's spawning.

For whatever reason, the spawner appears to have crashed the world, resulting in the included crash report. Having downgraded to 13w19a, I believe it has something to do with the new attributes system.

To create the spawner, I used this tutorial: https://www.youtube.com/watch?v=uX0ATqZClGo
The example villager was replaced with the aforementioned skeleton.