Mega Roth from 401(k) for solo LLC?

[johnhkc]

I read about the mega backdoor Roth stuff (move after-tax 401k to a Roth IRA) here on e-r and would like to give it a spin. I do software consulting via a solo LLC. Schwab says after-tax contributions to an individual 401(k) is not possible.

Is this true of all individual 401(k)'s or is it a Schwab limitation?