从自定义类初始化对象时，initializer元素不是常量错误

i am following along(mostly, im playing around with different functionality) with sams teach yourself cocoa touch programming and im building a calculator but when i try to initialize an object of the calculatormodel class i get this error: " error: initializer element is not constant". can anyone explain this or give me a solution? thanks so muchi am following along(mostly, im playing around