JavaScript Can't call prototype function inside another

i've been looking for an answer but everything i find seems to be saying that i'm not doing anything wrong. I'm trying to call a prototype function inside another one, but i keep getting the error that it is not a function.

For some reason it only happens within one specific method (tick), the rest has no problem with the same methods.

Here is my code:

Timer.prototype.getTimeRemaining = function(){
  return this.startTime - this.currentTimeTimed;
}

Timer.prototype.updateElement = function(){
  document.getElementById("clock").innerHTML = this.getTimeRemaining();
  return true;
}

Timer.prototype.tick = function(){
  if (this.running){
    this.currentTimeTimed += timePassedInMilliseconds;
  }
  this.getTimeRemaining();
  this.updateElement();
}

So the error i get is in tick(): Uncaught TypeError: this.getTimeRemaining() is not a function.

While testing some more, i think i found out why: i think it is because the tick function is called from an interval. I'm not sure, but that's the only thing i can think of that's different from the other functions. But i don't know how i should change my code then, if it is caused by the interval.

Thanks in advance.