i cant get minecraft to download 13w21a and start up

Put the summary of the bug you're having here

What I expected to happen was for mcd to download the snapshot 13w21a and start up
Describe what you thought should happen here

What actually happened was it showed the mojang screen then crashed
Describe what happened here

Steps to Reproduce:
1. click on version selection "snapshot 13w21a"
2. press play