Put the summary of the bug you're having here
What I expected to happen was...:
The world would load and work normally
What actually happened was...:
The moment i loaded the world it crashed
Steps to Reproduce:
1. Spawn a mob
2. Put a potion effect on it
3. turn it into a spawner
Based on a crash report provided in this issue, I believe with high certainty that this report may be a duplicate of MC-17399. You can view all information known about this specific crash at MCX-659.