No Error At Compile Time But Whenever I Try To Search A Data I Have Saved Through Id Error Is Shown In The Salary Text Box About Input String Was Not In He Correct Format

struct customer
       {
           public string name;
           public int id;
           public int salary;

       }

       customer[] cus = new customer[1];
       public int index = 0;

       private void Form1_Load(object sender, EventArgs e)
       {
       }


       private void button1_Click(object sender, EventArgs e)
       {

           cus[index].name = txtname.Text;

           cus[index].id = Convert.ToInt32(txtid.Text);
           cus[index].salary = Convert.ToInt32(txtsal.Text);

           index++;

           Array.Resize(ref cus, index + 1);

           MessageBox.Show("data saved");
           txtid.Text = " ";
           txtname.Text = " ";
           txtsal.Text = " ";


       }

       private void btnsearch_Click(object sender, EventArgs e)
       {
           {
               for (int i = 0; i < index; i++)
               {
                   if (cus[i].id == Convert.ToInt32(txtid.Text))
                   {
                       cus[i].name = txtname.Text;

                       cus[i].salary = Convert.ToInt32(txtsal.Text);

                       return;