Why does my code think that canvas functions are undefined in my constructor's prototype in JavaScript?

I have a constructor, Chomp, which creates a canvas element; its property that has the canvas element is this.c, but when I try to call functions on this.c in a method of my constructor, I receive an error: TypeError: Cannot read property 'beginPath' of undefined (line 28 in function Chomp.chomping). I need to know why this is and how to fix this problem. Below is the part of my code important to this problem.

  var Chomp = function(x, y) {
  /*many properties*/
  this.c = this.element.getContext("2d");
  /*a few more properties*/
}
Chomp.prototype.chomping = function() {
  /*some conditions,... properties like this.width were defined in Chomp, 
  but omitted because of their lack of importance to the question.*/
  this.c.clearRect(/*arguments*/);
  this.c.beginPath();
  this.c.moveTo(/*arguments*/);
  this.c.arc(/*arguments*/);
  this.c.lineTo(/*arguments*/);
  this.c.lineTo(/*arguments*/);
  this.c.fillStyle = "yellow";
  this.c.fill();
  /*more conditions*/
}

Thanks ahead of time! :)