Return nil in swift function

In Swift I have a function that returns some kind of object. That object is optional. When it does not exist, I suppose I should return nil, but Swift forbid me to do so. Following code is not working:

func listForName (name: String) -> List {

        if let list = listsDict[name] {
            return list
        }   else {
            return nil
        } 
    }

It says : error: nil is incompatible with return type 'List'

But I don't want to return something like empty List object, I want to return nothing when optional is empty. How to do that?