**Code** This code won't compile ```ts export type Errors<D> = { readonly [K in (keyof D | 'base')]?: string[] } class Model<D> { getErrors(): Errors<D> { return { base: ['some base error'] } } } ``` **Expected behavior:** It should compile without errors [Playground](https://www.typescriptlang.org/play/#src=export%20type%20Errors%3CD%3E%20%3D%20%7B%0A%20%20readonly%20%5BK%20in%20(keyof%20D%20%7C%20'base')%5D%3F%3A%20string%5B%5D%0A%7D%0A%0Aclass%20Model%3CD%3E%20%7B%0A%20%20getErrors()%3A%20Errors%3CD%3E%20%7B%0A%20%20%20%20return%20%7B%20base%3A%20%5B'some%20base%20error'%5D%20%7D%0A%20%20%7D%0A%7D)