Not defending/attacking the phrase "disastrously bad", but I'm not sure its a fair argument to say "the docs explain it", if the behavior isn't what a "reasonable" person would expect. Writing software that deletes your files (when you expect, say, the weather report) isn't excused if they write in the docs "oh this weather software randomly deletes files for no reason".
In this specific case though, even those docs do not explain what happens if the value is "invalid"!
In this specific case though, even those docs do not explain what happens if the value is "invalid"!