input string was not in a correct format?

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C#

Windows

hi am using windows form c# language.
while selecting from grid after clicking edit button all fields value should display in text related text box.but am getting error
"Input string was not in a correct format"

here is my code

private void tbSoldAmt_TextChanged(object sender, EventArgs e)
       {
           if ((!string.IsNullOrEmpty(tbSoldAmt.Text)) && (!string.IsNullOrEmpty(tbPurchaseAmt.Text)))
           {
               tbProfit.Text = (Convert.ToInt32(tbSoldAmt.Text) - Convert.ToInt32(tbPurchaseAmt.Text)).ToString();//this line am getting error
           }
       }

and here is btnEdit code

private void btnEdit_Click(object sender, EventArgs e)
       {
           int i;
           i = dataGridView1.SelectedCells[0].RowIndex;
           tbModeOfTransport.Text = dataGridView1.Rows[i].Cells[1].Value.ToString();
           tbCustomerName.Text = dataGridView1.Rows[i].Cells[2].Value.ToString();
           tbAddress.Text = dataGridView1.Rows[i].Cells[3].Value.ToString();

           tbFrom.Text = dataGridView1.Rows[i].Cells[4].Value.ToString();
           tbTo.Text = dataGridView1.Rows[i].Cells[5].Value.ToString();
           tbSeats.Text = dataGridView1.Rows[i].Cells[6].Value.ToString();
           tbFareAmount.Text = dataGridView1.Rows[i].Cells[7].Value.ToString();
           tbPurchaseAmt.Text = dataGridView1.Rows[i].Cells[8].Value.ToString();
           tbSoldAmt.Text = dataGridView1.Rows[i].Cells[9].Value.ToString();//in this line highlighted 
           tbProfit.Text = dataGridView1.Rows[i].Cells[10].Value.ToString();
           tbDate.Text = dataGridView1.Rows[i].Cells[11].Value.ToString();
           tbMobile.Text = dataGridView1.Rows[i].Cells[12].Value.ToString();




       }

can any one suggest me ?thank u