I... what... how? How do I get an error that there is no valid number for? And how does it differ from undefined variable? And why isn't it decided during preprocessing?

You're forgetting to return a value ;) Its not an error because there's nothing for the preprocessor to check... or something :p

I think you can also get this by passing an empty parameter to a function and trying to use it. SB has no way to police runtime types at compile-time

I saw that. Still an odd name for "bro, you forgot to return a value". Maybe there are other things that return the same error? Besides not passing an argument.