Multiple Contacts Issue in SpriteKit Game - skspritenode

Question

In my game, I have a ball, walls and gaps between these walls. Ball and walls are SKSpriteNode, gap is SKNode. I use gap to count the score.
I have the following code to handle the contacts and collisions. I checked plenty of documents and tutorials but cant find why there are multiple gap contacts happen when my ball contacts the gap. It prints "gap contact" for 6 times every time.
Bodytypes definitions:
enum BodyType:UInt32 {
case ball = 1
case wall = 2
case gap = 4
}
Ball:
var ball = SKSpriteNode(texture: SKTexture(imageNamed: "img/ball.png"))
ball.position = CGPointMake(frame.width/2, 50)
ball.physicsBody = SKPhysicsBody(texture: ball.texture, size: ball.size)
ball.physicsBody?.dynamic = true
ball.physicsBody?.allowsRotation = false
ball.physicsBody?.categoryBitMask = BodyType.ball.rawValue
ball.physicsBody?.collisionBitMask = BodyType.wall.rawValue
ball.physicsBody?.contactTestBitMask = BodyType.gap.rawValue | BodyType.wall.rawValue
Walls (walls are created through scheduledTimerWithTimeInterval every 2 seconds):
var wall1 = SKSpriteNode(texture: nil, color: UIColor.blackColor(), size: CGSizeMake(frame.size.width, 20))
var wall2 = SKSpriteNode(texture: nil, color: UIColor.blackColor(), size: CGSizeMake(frame.size.width, 20))
let gapSize = 70
let gapMargin = 50
let gapPoint = CGFloat(arc4random_uniform(UInt32(frame.size.width) - gapSize - (gapMargin*2)) + gapMargin)
wall1.position = CGPointMake(-wall1.size.width/2 + gapPoint, self.frame.size.height)
wall2.position = CGPointMake(wall1.size.width/2 + CGFloat(gapSize) + gapPoint, self.frame.size.height)
wall1.physicsBody = SKPhysicsBody(rectangleOfSize: wall1.size)
wall1.physicsBody?.dynamic = false
wall2.physicsBody = SKPhysicsBody(rectangleOfSize: wall2.size)
wall2.physicsBody?.dynamic = false
wall1.physicsBody?.categoryBitMask = BodyType.wall.rawValue
wall2.physicsBody?.categoryBitMask = BodyType.wall.rawValue
Gap Node:
var gap = SKNode()
gap.physicsBody = SKPhysicsBody(rectangleOfSize: CGSizeMake(CGFloat(gapSize), wall1.size.height))
gap.position = CGPointMake(CGFloat(gapSize)/2 + gapPoint, self.frame.size.height)
gap.runAction(moveAndRemoveWall)
gap.physicsBody?.dynamic = false
gap.physicsBody?.collisionBitMask = 0
gap.physicsBody?.categoryBitMask = BodyType.gap.rawValue
Contact Handling:
func didBeginContact(contact: SKPhysicsContact) {
let contactMask = contact.bodyA.categoryBitMask | contact.bodyB.categoryBitMask
switch(contactMask) {
case BodyType.ball.rawValue | BodyType.gap.rawValue:
println("gap contact")
default:
println("wall contact")
movingObjects.speed = 0
explosion(ball.position)
gameOver = 1
}
}
Wall collision works perfectly and prints "wall contact" only once when I hit the wall but I couldnt fix this multiple contact issue between the ball and gap.
UPDATE: The problem is with the ball's physicsbody. If I use circleOfRadius it works perfectly but then, this doesnt provide a great experience to the player as my ball shape is not a perfect circle so I want to use the texture to cover the physicsbody.