Overriding Methods

Hi,

I'm stuck with this challenge. Not sure if I did anything wrong.

Task : I've provided a base class Person in the editor below. Once an instance of Person is created, you can call fullName() and get a person's full name. 
Your job is to create a class named Doctor that overrides the fullName() method. Once you have a class definition, create an instance and assign it to a constant named someDoctor. 
For example, given the first name "Sam", and last name "Smith", calling fullName() on an instance of Person would return "Sam Smith", but calling the same method on an instance of Doctor would return "Dr. Smith".

Error : Bummer! Double check the implementation of the overridden method to match the directions specified

class Person {
  let firstName: String
  let lastName: String 

  init(firstName: String, lastName: String) {
    self.firstName = firstName
    self.lastName = lastName
  }

  func fullName() -> String {
    return "\(firstName) \(lastName)"
  }
}

class Doctor: Person {

    override func fullName() -> String {
        return "DR. \(lastName)"
    }
}

let someDoctor = Doctor(firstName: "Sam", lastName: "Smith")
someDoctor.fullName()