TS2345 error - Argument of type X is not assignable to parameter of type X[]

Question

export default class Foo {
  first: string;
  second: X[];
}

export function parse(foo?) {
  if(foo) {
    return Builder<Foo>(Foo)
      .first(foo.first || "")
      .second(XFunctions.parse(foo.second.map(second => XFunctions.parse(second))))
      .build();
  }

And I'm receiving the error as stated above: "argument of type X is not assignable to parameter of type X[]"

Here is the code for my X class:

export default class X {
  first: string;
  second: string;
  third: string;
  fourth: boolean
}

export function parse(x?) {
  if(x) {
    return Builder<X>(X)
      .first(x.first || "")
      .second(x.second || "")
      .third(x.third || "")
      .fourth(x.fourth)
      .build();
  }
  return Builder<X>(X)
    .first("")
    .second("")
    .third("")
    .fourth(true)
    .build();
}

I thought using map() would do the track as it would iterate over each element in X[], applying the parse function to each, but I must be misunderstanding how this works. I know this error is a common issue, and if another question I haven't seen better answers my question, I apologize in advance.