Programs terminates at return UIImage in Icon.toImage()

I have been trying to debug this but I can't put my finger on it: I have simplified my toImage function to the following: 

enum Icon {
    case ClearDay = "clear-day"
    func toImage() -> UIImage? {
        return UIImage(named: "\(self.rawValue).png")
    }
}

But every time I run the app, it stops at this return statement. Even with the code exactly as it was in the video, I keep getting this error. If I, however, created a function toImageString(), return the string and then construct my UIImage from that string, no error is thrown. Why is this?

func toImageName() -> String {
    return "\(self.rawValue).png";
}

Update: even if I return nil, which is allowed as it expects an optional, the same line is highlighted an my app terminates right then and there. The return statement seems to be the culprit, but I'm not seeing why.

I have also build a quick test case so I don't need to connect to the weather service, which goes like this:

var i = Icon.ClearDay;
println(i.toImageName()) 
// prints "clear-day.png"
var im = UIImage(named: i.toImageName()); 
// works
var im2 = i.toImage() 
// Terminates, prints (lldb) in green, marks `return` line with green, when commented out the next line works as expected.
self.currentWeatherImage?.image = im;