fatal error: unexpectedly found nil while unwrapping an Optional value (lldb)

import Foundation
import UIKit

struct Playlist {

    var title: String?
    var description: String?
    var icon: UIImage?
    var largeIcon: UIImage?
    var artists: [String] = []
    var backgroundColour: UIColor = UIColor.clearColor()

    init(index: Int) {
        let musicLibrary = MusicLibrary().library
        let playlistDictionary = musicLibrary[index]

        title = playlistDictionary["title"] as String!
        description = playlistDictionary["description"] as String!

        let iconName = playlistDictionary["icon"] as String!
        icon = UIImage (named: iconName)

        let largeIconName = playlistDictionary["largeIcon"] as String!
        largeIcon = UIImage(named: largeIconName)

        artists += playlistDictionary["artists"] as [String]

        let coloursDictionary = playlistDictionary["backgroundColour"] as [String: CGFloat]

        backgroundColour = rgbColourFromDictionary(coloursDictionary)
    }

    func rgbColourFromDictionary(colourDictionary: [String: CGFloat]) -> UIColor {

        let red = colourDictionary["red"]!
        let green = colourDictionary["green"]!
        let blue = colourDictionary["blue"]!
        let alpha = colourDictionary["alpha"]!

        return UIColor(red: red/255.0, green: green/255.0, blue: blue/255.0, alpha: alpha)

    }
}