Obtained the following traceback.  What I did was:

Enter root password.

Add a non-root user.

Noticed that I mistyped the user name.

Clicked on the username in the list box, to repopulate entry fields with
the particulars.

Edited the username field.

Hit 'Enter' until my change was reflected in the userlist box.

Hit 'Next', to go to the next screen.

Traceback.