Codable to Dictionary with nil value keys

Question

To convert Codable to Dictionary I have created this extension.

extension Encodable {
  func asDictionary() throws -> [String: Any] {
    let data = try JSONEncoder().encode(self)
    guard let dictionary = try JSONSerialization.jsonObject(with: data, options: .allowFragments) as? [String: Any] else {
      throw NSError()
    }
    return dictionary
  }
}

But I have special requirement like below.

struct Foo: Codable {
  let a: Foo1?
  let b: Foo2?
}
struct Foo1: Codable{
let x:String
}

struct Foo2: Codable{
let y:String
}

let foo: Foo = Foo(a:Foo1(x:"hello"),b: nil)
let dict = foo.asDictionary

Actual Output : {a:{x:hello}}

Excepted Output : {a:{x:hello,y:nil}}

How we can do this getting key with nil values?