TypeScript compilation error

[Joncom]

% tsc
decoder.ts:432:55 - error TS2339: Property 'error' does not exist on type 'Result<A, Partial<DecoderError>>'.
  Property 'error' does not exist on type 'Ok<A>'.

432             return Result.err(prependAt(`[${i}]`, nth.error));
                                                          ~~~~~


Found 1 error.

In this code block:

  static tuple<A>(decoders: Decoder<A>[]) {
    return new Decoder((json: unknown) => {
      if (isJsonArray(json)) {
        if (json.length !== decoders.length) {
          return Result.err({
            message: `expected a tuple of length ${decoders.length}, got one of length ${
              json.length
            }`
          });
        }
        const result = [];
        for (let i: number = 0; i < decoders.length; i++) {
          const nth = decoders[i].decode(json[i]);
          if (nth.ok) {
            result[i] = nth.result;
          } else {
            return Result.err(prependAt(`[${i}]`, nth.error));
          }
        }
        return Result.ok(result);
      } else {
        return Result.err({message: expectedGot(`a tuple of length ${decoders.length}`, json)});
      }
    });
  }