Cannot convert value of type 'String' to expected argument type 'SKNode'
<p>There is an issue on last line. How can I fix that? Issue says ''Cannot convert value of type 'String' to expected argument type 'SKNode'. '' Here is my code :</p>

<pre><code>import SpriteKit

let BallCategoryName = "ball"

class GameScene: SKScene {

    let ball = childNodeWithName(BallCategoryName) as! SKSpriteNode
</code></pre>