Input string was not in a correct format.

Hi expert, i have designed web form by using of stored procedure, i have salary column in decimal when i enter some values it working fine but in null condition getting error. input string was not in correct form. below is my code. plz help me out. thank you.

Line 62: obbll._conveyance=Convert.ToDecimal(txtconveyance.Text);//input string was not in correct form Line 63: obbll._ctcperyear=Convert.ToDecimal(txtctcperannum.Text);