I have been programming for up to 50 years and this is the strangest language I have ever encountered. And I have used a lot of them!
Well, that's the problem right there!
It's like that New Math song...
Inform 7 is definitely aggravating at times, and one of the worst problems it has is documentation. In my opinion the error messages are part of the documentation, and they're no exception.
Fortunately, this here forum more than makes up for the shortcomings of the documentation. I bet you'll have an answer to this by Monday afternoon.
The thing for me, besides the addictive yet oddly counter-productive thrill of learning a programming language that's not like any other I know, is that after hours of grinding my teeth and tearing my hair to get the syntax right, I have readable code that I can come back to later and still understand.