Unable to start local version

I have a local version set up in order to use a modded version of Minecraft. This was working fine in the previous version of the launcher but in version 0.9 it fails with:

Client> Error: Could not find or load main class net.minecraft.client.Minecraft
Game ended with bad state (exit code 1)

I have tried replacing the .jar file with a fresh, un-modified copy of release 1.5.2 and re-copying the .json file but I still get the same error.

The version folder, .jar file, .json file and the id field within the .json file are all the same (Village Info).

This is all under a separate profile that I set up specifically for this version. I also tried setting up a separate profile for the release 1.5.2 version directly and that works fine.