Sum Of textboxes values

Question

this is my code

 int sum = int.Parse(TextBox5.Text) + int.Parse(TextBox6.Text) + int.Parse(TextBox7.Text);

            Label1.Text = String.Format("{0}", sum);

when I enter all three textboxes it runs clearly but if i keep any one or two of them null it gives error...

input was not in correct formate ..

how can I solve this error.....