Input string was not in a currect format in cell validaing event in windows forms

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C#

C#4.0

good evening....

i have a problem...
ie.

i wrote a code in gridveiw_cell validating event..like below

private void gridStoreIssues_CellValidating(object sender, DataGridViewCellValidatingEventArgs e)
       {
           try
           {
               if (e.RowIndex != -1 && e.ColumnIndex == colIsuQty.Index && ((StoreTransaction)gridStoreIssues.Rows[e.RowIndex].DataBoundItem) != null)
               {
                   if (colIndQty.Visible && Convert.ToSingle(e.FormattedValue) > Convert.ToSingle(gridStoreIssues[colIndQty.Index, e.RowIndex].Value))
                   {
                       Console.Beep();
                       e.Cancel = true;
                       MessageBox.Show("Issue Qty Cannot Exceed Indent Qty");
                       StatusBar.Text = "Issue Qty Cannot Exceed Indent Qty";
                       gridStoreIssues.CurrentCell = gridStoreIssues[colIsuQty.Index, e.RowIndex];
                       gridStoreIssues.BeginEdit(true);
                   }
                   else if (Math.Round(Convert.ToSingle(e.FormattedValue), 3) > Math.Round(((StoreTransaction)gridStoreIssues.Rows[e.RowIndex].DataBoundItem).QuantityOnHand, 3))
                   {
                       Console.Beep();
                       DialogResult dr = MessageBox.Show(string.Format("The Issued qty is over the available qty of {0:N3}.", Math.Round(((StoreTransaction)gridStoreIssues.Rows[e.RowIndex].DataBoundItem).QuantityOnHand, 3)), "Continue", MessageBoxButtons.OK, MessageBoxIcon.Information);

                       if (dr == DialogResult.OK)
                       {
                           e.Cancel = true;
                           gridStoreIssues.BeginEdit(true);
                       }
                       else
                           e.Cancel = false;
                   }
               }
           }
           catch (Exception ex)
           {
              // StoreTransFactory.GenerateExceptionTextFile(ServerTime.Now + ",StoreIssueForm," + this.Name + "," + ex.Message + "," + ex.StackTrace + "\n", Application.StartupPath + @"\Store.txt");
           }
       }

its working fine in my system..
but in my clint system give a exception like bellow

16:31:08,StoreIssueForm,StoreIssueForm,Input string was not in a correct format., at System.Number.ParseSingle(String value, NumberStyles options, NumberFormatInfo numfmt)
at System.Convert.ToSingle(String value, IFormatProvider provider)
at System.String.System.IConvertible.ToSingle(IFormatProvider provider)
at System.Convert.ToSingle(Object value)
at Prosynergix.Store.StoreIssueForm.gridStoreIssues_CellValidating(Object sender, DataGridViewCellValidatingEventArgs e)

i dont know why it was gave exception...
any one plz help me...