If you set the world to the nether, and run start, it says generating for the overworld. I don't know if it is a display error, or if the command doesn't work.

v 1.3.92 Chunky (Forge)

Seems to work on my end. Make sure you haven't disabled the nether, and are using the correct dimension ID (e.g. "nether" is not valid, "the_nether" is).

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