Using R.map on a Struct in fp-ts

Question

Here is what I am trying to do:

import * as R from 'fp-ts/lib/Record';
type Ex<T extends Record<string, unknown>> = { [K in keyof T]: (n: number) => T[K] };

const f =
  <T extends Record<string, unknown>>(ex: Ex<T>) =>
  (n: number): T =>
    pipe(
      ex,
      R.map((e) => e(n)),
    );

The problem is, I get:

   Type 'keyof T' is not assignable to type 'string'.
          Type 'string | number | symbol' is not assignable to type 'string'.
            Type 'number' is not assignable to type 'string'.ts(2345)

If this were a monad, I could use Apply.sequenceS, but I don’t know to do the equivalent thing when it’s a simple function.