invalid byte sequence in UTF-8

[lilydjwg]

If there are some files with invalid characters ruby raises an error:

invalid byte sequence in UTF-8
(eval):91:in `gsub'
(eval):91:in `regex_escape'
(eval):595:in `highlight_selected_index'
(eval):575:in `refresh'
(eval):485:in `run'
(eval):759:in `run'
(eval):768:in `run_from_path'
(eval):1:in `block in <main>'
(eval):238:in `profile'
(eval):1:in `<main>' 

I have seen this kind of errors before on Windows with codepage 936 (cp936), but I did not find out exactly how to triger this error.