Vector/Array sorting with custom compare function bug

Hi,

can someone help me to understand custom compare function behavior when sorting Vector or Array of custom objects? The problem I have is really strange and I made small test which I'll share now.

I have created Vector (and tried with Array also later, it's the same) with 5 simple objects. Objects have only 2 variables, "a" and "b". All Objects have the "a" value equal to 1 and "b" values are consecutive numbers from 1 up to 5. Custom compare function compares and sorts these objects on "a" variable and here I have strange behavior. With every call of sort function using this custom compare function 1st and middle object (in 5 element vector it's 3rd, in 10 element vector it's 6th and so on) replace them selves in Vector object.

I have put the code below:

var objects:Vector.<Object> = new <Object>[{a:1, b:1}, {a:1, b:2}, {a:1, b:3}, {a:1, b:4}, {a:1, b:5}];

function sortVector(a:Object, b:Object):int
{
  if(a.a < b.a)
       return -1;
  else if(a.a > b.a)
       return 1;

  return 0;
}

var i:uint;
for(i=0; i<objects.length; i++)
  trace(objects.b);
//Output: 1, 2, 3, 4, 5

objects.sort(sortVector);

for(i=0; i<objects.length; i++)
  trace(objects.b);
//Output: 3, 2, 1, 4, 5

objects.sort(sortVector);


for(i=0; i<objects.length; i++)
  trace(objects.b);
//Output: 1, 2, 3, 4, 5

It's strange for me that sort function exchange two elements when they have equal compare values. Custom compare function above will actually always return 0 which means that elements have same compare values. And if this is regular behavior, why it doesn't exchange all other elements?

Does anyone have idea what's going on here?