Input string was not in a correct format.

Question

When i trying to convert string to double in ADO.Net.. I got the Error " Input string was not in a correct format." Please anyone tell answer me..

My code is 

 string sFee = Convert.ToString(serviceFee.Text);

                 string tFee = Convert.ToString(typingFee.Text);

                 string gFee = Convert.ToString(govermentFee.Text);

                 double ssFee = Convert.ToDouble(sFee);

                 double ttFee = Convert.ToDouble(tFee);

                 double ggFee = Convert.ToDouble(gFee);