What am I missing?

I have been struggling with this module for weeks, and read the readme that I almost know it by heart. In your step:

D) The fourth tab only shows up if you've enabled the accounttypes module. The tab is labeled
'Tie account type to membership' (admin/user/nf_registration_mod/account_type). On this page
you can set a default account type to be given to a user whenever their membership type
changes to that membership type.
I) This is a straight-forward page where each membership type is displayed and there is a
drop down next to each to choose which account type to associate with it. Select the one
you want and click 'Save assignments'.

This is where I keep getting the error "The account type is not valid. You must choose from the dropdown list."

What have I missed?