Put the summary of the bug you're having here

What I expected to happen was...:
Describe what you thought should happen here
I didn't expect it to crash, that's for sure.

What actually happened was...:
Describe what happened here
The crash happened when I was testing sand variations. It happened right around issueing this command:
/setblock ~ ~-1 ~ minecraft:sand 15 replace

Steps to Reproduce:
1. Place a block of sand (or any block, really)
2. stand on top of it.
3. type "/setblock ~ ~-1 ~ minecraft:sand 15 replace"

See attached crash report.