ASP.Net Getting error second time: Input string was not in a correct format.

when I adding for first time it's added successfully but for the second time it raises this error:

Input string was not in a correct format.

protected void btn_save_add_Click(object sender, EventArgs e) Line 147:        { Line 148:            pro.Pro_ID = Convert.ToInt32(txt_prodID_add.Text); Line 149:            pro.Pro_Name = txt_prod_name_add.Text; Line 150:            pro.CategoryProd_ID = Convert.ToInt32(ddl_ctegOf_Product_add.SelectedValue);