Enumerating Object.keys() in node.js - strange issue

There is an issue with enumerating Object.keys() in node.js that I do not understand. With the following code:

Object.prototype.tuple = function() {
  var names = Object.keys(this);
  console.log("Dump of names:");
  console.log(names);
  console.log("FOR loop using indexes:");
  for (var k = 0; k < names.length; k++)
  {
    console.log(names[k]);
  }
  console.log("FOR loop using enumeration:");
  for (var z in names)
  {
    console.log(z);
  }
  return this;
};

var x = {a:0, b:0, c:0}.tuple();

I get the following results on the console:

Dump of names:
[ 'a', 'b', 'c' ]
FOR loop using indexes:
a
b
c
FOR loop using enumeration:
0
1
2
tuple

Could somebody explain where does an extra "tuple" come from in the second loop? While defined as function in Object.prototype, it is neither an own property of x object, nor included in names array.

I am using node.js version 0.8.20.