Assert in Monster Battle Arena when fighting Iron Golem

Verification builds: Release 880513 - 1.5 BETA
Summary: In Monster Battle Arena there is an assert when spawning an iron golem.

Repro Steps:
1. Download Monster Battle Arena from the store
2. do /summon iron_golem

Observed Results: Something in the Iron Golems is causing an assert which appears when an iron golem exists in the world. The game doesn't hard crash though.

Expected Results: Shouldn't be an assert.

Impact: As part of Monster Battle Arena the iron golems are a combatant. They are in waves mode and used as opponents.

Screenshot