Typescript get object keys whose value is string

Question

Is it possible to get the keys of an object whose values are string? I have an object MyType like:

type SomeObjectType = {
    a: number;
    b: string;
};

type MyType = {
    a: string;
    b: string;
    c: number;
    d: SomeObjectType;
};

Expected result "a" | "b":

type Test = OnlyStringKeys<MyType, keyof MyType>; // "a" | "b"

My try:

type OnlyStringKeys<T, K extends keyof T> = T[K] extends string ? K : never;
type Test = OnlyStringKeys<MyType, keyof MyType>; // never

Playground