Mapped type based on interface method is empty

[panstromek]

When trying to generate schema for mapped type like this, I get just an empty object:

export interface Functional {
    fun(): string
}

export type FunctionalReturnTypes = {
    [Key in keyof Functional]: ReturnType<Functional[Key]>
}

Note: even if I replace the ReturnType<Functional[Key]> with just string, the schema is empty, so it seems like the culprit is that the base interface is functional.