＆＃39; __＆＃39;不能转换为＆＃39; StringLiteralConvertible＆＃39;

Question

我不明白为什么这段代码会产生错误：

Alamofire.request(Alamofire.Method.PATCH, "https://example.herokuapp.com/", parameters: [
            "format":"json",
            "todo":
                ["isComplete":todo?.isComplete],
            "id": todo?.id // <--- ERROR: is not convertible to 'StringLiteralConvertible'
            ]
        )