Wow, that is an unhelpful error message. It could have told you it was expecting a number. It turns out that -i is short for "interval" and expects a number, whereas -I is used to specify an interface.
It’s actually a technical term in language theory – a term that cannot be produced by the language, but is still considered part of the ‘universe’ (i.e., the set of all terms) is called ‘garbage’ or ‘junk’.
Since I can’t find a source online to verify this claim, this might just have been the case in my courses…
"Invalid" or "unparseable" are more understandable descriptors in normal language. I don't think I ever heard of garbage/junk being used for that in language theory but it may be domain specific usage.