Actionscript 3.0：错误＃1006：value不是函数

我是ActionSccript 3.0的初学者 . 我做了这个代码工作正常但是当我把数量超过150克时，我在输出中出现错误，错误＃1006：值不是函数请帮助 . 介意给我正确的代码？谢谢

//import controls
import fl.controls.RadioButtonGroup;
import flash.events.MouseEvent;
import fl.controls.RadioButton;
// This line makes the button, btnCalculate wait for a mouse click
// When the button is clicked, the determineCost function is called
btnCalculate.addEventListener(MouseEvent.CLICK, determineCost);

// These lines make the textinput and the radioButtons wait for a mouse click
// When these components are clicked, the clearLabels function is called
txtinMass.addEventListener(MouseEvent.CLICK, clearLabels); 

// This is the determineCost function
// e:MouseEvent is the click event experienced by the button
// void indicates that the function does not return a value
function determineCost(e:MouseEvent):void
{
    // declare the variables
    var Mass:uint;
    var Letter:RadioButtonGroup = new RadioButtonGroup("Letters");

    // put the groups together
    FirstRadio.group = Letter;
    SecondRadio.group = Letter;

    // get the mass from the user
    Mass = uint(txtinMass.text); 

    // determine the cost

    if (Mass <= 30 && Letter.selection.label == "First Class") 
    { 
        lblCost.text = "0.38"; 
    }
    else if (Mass <= 30 && Letter.selection.label == "Second Class")
    {
        lblCost.text = "0.28";
    }
    else if (Mass > 30 && Mass <= 50 && Letter.selection.label == "First Class")
    {
        lblCost.text = "0.55";
    }
    else if (Mass > 30 && Mass <= 50 && Letter.selection.label == "Second Class")
    {
        lblCost.text = "0.40";
    }
    else if (Mass > 50 && Mass <= 100 && Letter.selection.label == "First Class")
    {
        lblCost.text = "0.73";
    }
    else if (Mass > 50 && Mass <= 100 && Letter.selection.label == "Second Class")
    {
        lblCost.text = "0.55";
    }
    else if (Mass >= 150 && Letter.selection.label == "First Class")
    {
        lblCost = ((0.73 + 0.24 * Math.floor((Mass - 100) / 50)))();
    }
    else if (Mass >= 150 && Letter.selection.label == "Second Class")
    {
        lblCost = ((0.55 + 0.19 * Math.floor((Mass - 100) / 50)))();
    }
}

// This is the clearLabels function 
// e:MouseEvent is the click event experienced by the textInput and the radioButtons 
// void indicates that the function does not return a value 
function clearLabels(e:MouseEvent):void 
{ 
    lblCost.text = ""; 
    txtinMass.text = ""; 
}

我很确定错误是在我的最后2个if if语句中 . 当我把质量超过150克时，我试图让方程的答案出现 .