Swift protocol defining class method returning self

I had code that was working in XCode 6 beta but stopped working recently after updating to xcode 6.1.

This is my protocol:

protocol CanDeserialiseFromJson {
    class func FromJson(json : JSONValue) -> Self
}

This is implementation:

extension Invoice : CanDeserialiseFromJson {
    class func FromJson(json : JSONValue) -> Self {
        return Invoice()
    }
}

This fails giving error:

'Invoice' is not convertable to 'Self'

As I said, this used to work and I can't work out why it doesn't anymore