Type Error for hit testing two objects created with actionscript

I am making a zombie shooter game where I get an error when I try to hit test two objects created with actionscript. Could someone post a possible fix please.

Here is the Error Code;

TypeError: Error #2007: Parameter hitTestObject must be non-null.
    at flash.display::DisplayObject/_hitTest()
    at flash.display::DisplayObject/hitTestObject()
    at game3_fla::MainTimeline/zombiemove()

and here is my game code;

import flash.events.Event;
var zombiehits:Number = 2;
var numzombies1:Number = GenerateRandomNumber(10);
var zombie : Z = new Z();
var objz;
var objb;
stop();
//Change mouse
crosshair_mc.mouseEnabled = false;
crosshair_mc.addEventListener(Event.ENTER_FRAME, fl_CustomMouseCursor);
Mouse.hide();
//EventListners;
stage.addEventListener(Event.ENTER_FRAME, alwayson);
stage.addEventListener(MouseEvent.MOUSE_MOVE,facecursor);
stage.addEventListener(MouseEvent.MOUSE_DOWN, mouseDown);
//Code;
var startcountdown:Number = 5;
var fl_CountDownTimerInstance:Timer = new Timer(1000,startcountdown);
fl_CountDownTimerInstance.addEventListener(TimerEvent.TIMER_COMPLETE, startgame);
fl_CountDownTimerInstance.start();

function createZombie():void
{
    zombie.rotation = 90;
    zombie.height = 85.35;
    zombie.width = 55.9;
    zombie.x = 403.25;
    zombie.y = -86.9;
    addChild(zombie);
    trace("spawned the zombie");
    zombie.addEventListener(Event.ENTER_FRAME,zombiemove);
}

function zombiemove(event:Event)
{
    var zombie = event.currentTarget;
    objz = zombie;
    zombie.y +=  1;
    if (zombie.hitTestObject(objb) == true)
    {
        dead -=  1;
    }
    if (dead == 0)
    {
        removeChild(zombie);
        zombie.removeEventListener(Event.ENTER_FRAME,zombiemove);
    }

}

function alwayson(event:Event)
{
    if (character_mc.hitTestObject(wallleft) == true)
    {
        character_mc.x +=  speedchar;
    }
    if (character_mc.hitTestObject(wallright) == true)
    {
        character_mc.x -=  speedchar;
    }
    if (character_mc.hitTestObject(walltop) == true)
    {
        character_mc.y +=  speedchar;
    }
    if (character_mc.hitTestObject(wallbottom) == true)
    {
        character_mc.y -=  speedchar;
    }
}

function fl_CustomMouseCursor(event:Event)
{
    crosshair_mc.x = stage.mouseX;
    crosshair_mc.y = stage.mouseY;
}

function facecursor(event):void
{
    character_mc.rotation = (180 * Math.atan2(mouseY - character_mc.y,mouseX - character_mc.x))/Math.PI;

}

function mouseDown(event)
{
    var b = new Bullet();
    b.x = character_mc.x;
    b.y = character_mc.y;
    b.angleRadian = Math.atan2(mouseY - character_mc.y,mouseX -character_mc.x);
    b.addEventListener(Event.ENTER_FRAME, bulletEnterFrame);
    addChild(b);
}



function bulletEnterFrame(event)
{
    var b = event.currentTarget;
    objb = event.currentTarget;
    b.x +=  Math.cos(b.angleRadian) * speedb;
    b.y +=  Math.sin(b.angleRadian) * speedb;
    b.rotation = b.angleRadian * 180 / Math.PI;
    if (b.hitTestObject(objz) == true)
    {
        removeChild(b);
        b.removeEventListener(Event.ENTER_FRAME, bulletEnterFrame);
    }
    if (b.x < 50 || b.x > 675 || b.y < 50 || b.y > 500)
    {
        removeChild(b);
        b.removeEventListener(Event.ENTER_FRAME, bulletEnterFrame);
    }
}
character_mc.addEventListener(Event.ENTER_FRAME, fl_MoveInDirectionOfKey);
stage.addEventListener(KeyboardEvent.KEY_DOWN, fl_SetKeyPressed);
stage.addEventListener(KeyboardEvent.KEY_UP, fl_UnsetKeyPressed);

function fl_MoveInDirectionOfKey(event:Event)
{
    if (upPressed)
    {
        character_mc.y -=  speedchar;
    }
    if (downPressed)
    {
        character_mc.y +=  speedchar;
    }
    if (leftPressed)
    {
        character_mc.x -=  speedchar;
    }
    if (rightPressed)
    {
        character_mc.x +=  speedchar;
    }
}

function fl_SetKeyPressed(event:KeyboardEvent):void
{
    switch (event.keyCode)
    {
        case Keyboard.UP :
            {
                upPressed = true;
                break;



            };
        case Keyboard.DOWN :
            {
                downPressed = true;
                break;



            };
        case Keyboard.LEFT :
            {
                leftPressed = true;
                break;



            };
        case Keyboard.RIGHT :
            {
                rightPressed = true;
                break;



        }
    }
};

function fl_UnsetKeyPressed(event:KeyboardEvent):void
{

    switch (event.keyCode)
    {
        case Keyboard.UP :
            {
                upPressed = false;
                break;



            };
        case Keyboard.DOWN :
            {
                downPressed = false;
                break;



            };
        case Keyboard.LEFT :
            {
                leftPressed = false;
                break;



            };
        case Keyboard.RIGHT :
            {
                rightPressed = false;
                break;



        }
    }
};


function GenerateRandomNumber(limit:Number):Number
{
    var randomNumber:Number = Math.floor(Math.random() * (limit + 1));
    return randomNumber;
}

function startgame(event:Event)
{
    createZombie();
    trace("complete");
}