*What I expected to happen was that the fill command would replace the lava with air.

Command: /fill ~5 ~4 ~5 ~-5 ~-4 ~-5 air 0 replace lava

*What actually happened was that the lava began to spread, as if the command hadn't worked.

Steps to Reproduce:
1. Go into a world from 14w28b.
2. Place a bucket of lava down and type in the command
3. /fill ~5 ~4 ~5 ~-5 ~-4 ~-5 air 0 replace lava