Put the summary of the bug you're having here

A superflat world to be loaded with the bottom layer bedrock and the next layer be farmland.
Describe what you thought should happen here

My game crashed.
Describe what happened here

Steps to Reproduce:
1. Create a new world.
2. Make the world type "Superflat."
3. Make it customized to have the bottom layer be bedrock and the next layer be farmland.
4. Create the world.
5. It crashes.