when i press clear button error occurs

Question

  double days = Convert.ToDouble(txt_working.Text);
  double sundays = Convert.ToDouble(txt_days.Text);
  double a = Convert.ToDouble(txt_basicscale.Text);
  txt_basicamount.Text = Convert.ToDouble((a / days) * sundays).ToString();


the above code is working but when i press clear button error occurs in first line

input string was not in a correct format.

clear button code as follows

txt_wroking.text="";
txt_days.text="";
txt_basicscale.text="";
txt_basicamount.text="";


i dont know.please tell the correct thats why i am asking.