Input String was not in correct Format. Please Help

Question

This is the line of code where i got the error:

private void txtTaxPer_TextChanged(object sender, EventArgs e)

{

try

{

if (string.IsNullOrEmpty(txtTaxPer.Text))

{

txtTaxAmt.Text = "";

txtTotal.Text = "";

return;

}

txtTaxAmt.Text = Convert.ToInt32((Convert.ToInt32(txtSubTotal.Text) * Convert.ToDouble(txtTaxPer.Text) / 100)).ToString() ;

txtTotal.Text = (Convert.ToInt32(txtSubTotal.Text) + Convert.ToInt32(txtTaxAmt.Text)).ToString();

}

catch (Exception ex)

{

MessageBox.Show(ex.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);

}

}