Error when transforming record keys in a schema

S.Struct({
  items: S.Record({
    key: S.String.pipe(
      S.transform(S.String, {
        decode: (key) => key.replaceAll(" ", ""),
        encode: Function.identity,
        strict: true,
      }),
      S.asSchema
    ),
    value: S.Struct({
      quantity: S.NullishOr(S.Int.pipe(S.nonNegative())),
      volume: S.NullishOr(S.Int.pipe(S.nonNegative())),
    }),
  }),
});
error: Uncaught (in promise) Error: Unsupported key schema
schema (Transformation): (string <-> string)