Issue with `ReadonlyArray.reduce` and widening type of `Either`/`Effect`

declare const advanceOutputOrFail: (value: number) => (output: number) => Either.Either<string, number>;

ReadonlyArray.reduce(
    [1, 2, 3, 4],
    Either.right(0),
                           // TS Error: Either<string, number>' is not assignable to type 'Either<never, number>
    (output, nextValue) => Either.flatMap(output, advanceOutputOrFail(nextValue)),
);