If a masterlist and prelude are loaded together, the prelude's content replaces the masterlist's embedded prelude, and that causes line numbers in subsequent parsing errors to not match the masterlist file's line numbers.

It would be good if the line numbers could somehow be adjusted to undo the effect on them of the prelude replacement, but failing that it would also help to at least log how much of the masterlist was replaced and how long the replacement prelude is.