Xcode Collision Detection Error Cocos2D Assertion Error - cocos2d-iphone

Question

I am making a ball game in cocos2D, where you can slide the ball in a direction, and it will travel until it collides with a wall. My collision detection is giving me an "assertion error" when I swipe the ball in a direction.
I am using tiledmap to make the levels.
The way I've coded it is that I find the tilesize that my ball is in, then, in a for loop, I check if there are any wall sprites around the ball.
Any ideas where its going wrong?
-(void)callEveryFrame:(ccTime)dt{
//Error Here vvv (Assertion Failure in -[CCTMXLayer tileAt:])
NSLog(#"First Touch x: /%f Last Touch x: /%f First Touch y: /%f Last Touch y: /%f", firstTouch.x, lastTouch.x, firstTouch.y, lastTouch.y);
if(swipeLeft || swipeRight || swipeDown || swipeUp)
{
bool collision = false;
int ballTileX = (player.position.x / level1.tileSize.width);
int ballTileY = (player.position.y / level1.tileSize.width);
int collisionAreaX = ballTileX-1;
int collisionAreaY = ballTileY-1;
for(int i = 0; i < 9; i++)
{
bool tilePresent = [background tileAt:CGPointMake(collisionAreaX, collisionAreaY)] != NULL;
if(tilePresent && CGRectIntersectsRect(CGRectMake(collisionAreaX*level1.tileSize.width, collisionAreaY*level1.tileSize.height, level1.tileSize.width, level1.tileSize.height), CGRectMake(ballTileX*level1.tileSize.width, ballTileY*level1.tileSize.height, level1.tileSize.width, level1.tileSize.height)))
{
collision = true;
break;
}
if(i % 3 == 0)
{
collisionAreaX -= 2;
collisionAreaY++;
}
else
{
collisionAreaX++;
}
}
if(collision) {
swipeLeft = false;
swipeRight = false;
swipeDown = false;
swipeUp = false;
}
}
}