gvim options / Settings window cannot write buftype is set e382

A serious case of RTFM, on my account. The top few lines explain how to edit it.

" Each "set" line shows the current value of an option (on the left).
" Hit <CR> on a "set" line to execute it.
"            A boolean option will be toggled.
"            For other options you can edit the value.
" Hit <CR> on a help line to open a help window on this option.
" Hit <CR> on an index line to jump there.
" Hit <Space> on a "set" line to refresh it.

I then pressed Enter, and my setting took affect immediately

on line 155
lines   number of lines in the display
    set lines=50