Weird throw for indexOf with Array

I am developing an application that spawns a world, and there are caves in the world, I am trying to check if there is a cave block(blank area) above specified coordinates which are in my array, I am checking if indexOf(coordinates) are equal to current coordinates - 1 Ycoordinate. Here is my code: (things I am talking about occur in lines 4-14.)

private function checkCave():void
        {

            if (caveAlready == false)
            {    
                var blockYMinus = (blockY - 1)
                var aboveIndex = ""
                    aboveIndex = i + "," + (blockYMinus)
                var caveAbove = ""
                    caveAbove = caveLocations.indexOf(aboveIndex)
                    trace(caveAbove)
                if (caveAbove == 0)
                {
                    caveAlready = true; trace("LOCATION WITH CAVE ABOVE: " + i + "," + blockY)
                }
            }
            if (caveAlready == false)
            {
                var caveRandom:Number = Math.round(Math.random() * 2000 + (caveSpace * .5));
                if (caveRandom <= 30)
                {
                    Global.vars.cave = true;
                    caveAlready = true;
                }
            }
            else
            {
                caveRandom = Math.round(Math.random() * 33);
                if (caveRandom <= 30)
                {
                    Global.vars.cave = true;
                    caveAlready = true;
                    caveLocations.push(i + "," + blockY);
                }
                else
                {
                    Global.vars.cave = false;
                    caveAlready = false;
                }

In line 11, I trace the value of caveAbove, which should only be -1 or 0, but in my output, I get these values and many more up into the thousands:

-1

-1

-1

-1

-1

-1

-1

-1

-1

0

LOCATION WITH ABOVE: 36,10

-1

-1

-1

-1

-1

-1

15

30

31

32

33

34

35

36

37

38

39

40.

My main concern is that I only got a throw of 0 once? And cannot seem to get it more than once... I don't know if I am using this indexOf properly, so please point me in the right direction.