You used forge 977, which is a early beta version. Try again with the newest version, and i'm sure it will work.
Note: Forge is not done yet and can contain crashing or even world corrupting bugs. Be careful with using non-recommended versions
To do this, override the following function in your block class:
public TileEntity createNewTileEntity(World world, int meta)
This should return an TileEntity based on the block metadata.