Custom transform class outside the framework

I would like to create the following custom transform class for my project.

~~~~
class URLTransform<ObjectType, JSONType>: MapperTransform<ObjectType, JSONType> {
        
    override func transformFromJSON(value: AnyObject?) -> ObjectType? {
        if let str = value as? String {
            return (NSURL(string: str) as ObjectType)
        }
        return nil
    }
    
    override func transformToJSON(value: ObjectType?) -> JSONType? {
        if let url = value as? NSURL {
            return (url.absoluteString as JSONType)
        }
        return nil
    }
}
~~~~

However, this causes compilation error as follows:

    'URLTransform<NSURL, String>' cannot be constructed because it has no accessible initializers