Realms crashes when putting Horses through portals

While on my realms world, I am moving horses to my spawn house using the nether. Just as the horse goes through the portal to the nether, the game crashes with the attached Java error. I then have to completly restart minecraft to get back into the world. The horse I was sending to the nether is also deleted.

This doesn't happen every time I put horse through the portal. The last time I was on the sixth horse. I could not repeat this in a single player world.