I found an issue with your usage of mapped types to output a string literal union type whereby it cannot be used safely in as many places as a "regular" string literal union type. I don't think it's the fault of this library, but it's a pain to debug so I'm filing this issue to bring it your attention. I've filed more details atmicrosoft/TypeScript#16553, as I think it's a bug in the compiler. Perhaps a caveat in the readme is in order?