How to validate map with dynamic keys (FormikErrors<T>)

[tparvi]

I am trying to validate form which uses dynamic keys but FormikErrors thinks these keys should be strings.

As you can see in this codesandbox I cannot return error message for field foo:
https://codesandbox.io/s/formik-errors-dynamic-keys-7pe2v

type Day = "Monday" | "Tuesday";
type Schedule = {
  foo: string;
};
type WeekdayMap<Schedule> = {
  [weekday in Day]?: Schedule;
};

const initialValues: WeekdayMap<Schedule> = {
  Monday: {
    foo: "anything"
  }
};

 function validate(
    values: WeekdayMap<Schedule>
  ): FormikErrors<WeekdayMap<Schedule>> {
     // TypeScript thinks Monday should be string and says Type '{ foo: string; }' is not assignable to type 'string'
    // How do I return error for field foo which is inside Monday?
    return {
      Monday: {
        foo: "I want to return error for this field"
      }
    };
  }

[tparvi]

Got some help and it seems I need to do the following to get around TS error:

  function validate(
    values: WeekdayMap<Schedule>
  ): FormikErrors<WeekdayMap<Schedule>> {
    // Without this TS says : Type '{ foo: string; }' is not assignable to type 'string'
    const Monday = "Monday" as Day;
    return {
      [Monday]: {
        foo: "I want to return error for this field"
      }
    };
  }