Input string was not in a correct format

Getting Input String Was not in a correct Format
.Actually I had written a method in webservice as method name as caculator
 [WebMethod]
   public int caculator(int firstNumber,int secondNumber,string operation)
    {
        int result = 0;
        switch(operation)
        {
            case "+": result = firstNumber + secondNumber;
                break;
            case "-": result = firstNumber - secondNumber;
                break;
            case "*": result = firstNumber * secondNumber;
                break;
            case "/":result = firstNumber / secondNumber;
                break;
        }
        return result;
    }
and Iam consuming this in webapplication where I have written as
private void BindData()
        {
            string operation = txtOperation.Text;
            WebService1 objWebService = new WebService1();
            int result=objWebService.caculator(int.Parse(txtFirstNumber.Text), int.Parse(txtSecondNumber.Text), operation);
            if (result != 0)
                Response.Write(result);
        }
 in this iam getting error at this line
int result=objWebService.caculator(int.Parse(txtFirstNumber.Text), int.Parse(txtSecondNumber.Text), operation);

i doesn't understand why iam getting error here. Even though i converted it into string
could you please suggest as soon as possible