Cannot read property error when calling a member function

Question

I have the following module Dog. I want to call the member function wagTail() every time I call walk(), because it is a happy dog.

class Dog {
  constructor(name){
     this.name = name
  }

  walk(){
    doAsyncStuffToMoveLegs((error) => {
      if(!error) {
        this.wagTail()
      }
    })
  }

  wagTail(){
    // initiate tail wagging :)
  }
}
module.exports = {Dog: Dog}

I get the following error: TypeError: Cannot read property 'wagTail' of undefined. What am I doing wrong?