formatting a decimal inside repeater

Hi,
I have the repeater below.

The value that InvoiceAmount is giving is:
512.33000 or whatever - it is a decimal.

I need to format it to 2 decimal place.

I have tried:
<%#DataBinder.Eval(Container.DataItem, "InvoiceAmount").ToString("#00.00")%>

but get:
 System.FormatException: Input string was not in a correct format.

What am I doing wrong?

<asp:Repeater ID="rptResults" runat="server" >                                       
                        <ItemTemplate>
                        <tr>
                            <td>                                
                                <%#DataBinder.Eval(Container.DataItem, "RegNumber")%>
                            </td>
                            <td>
                                <%#DataBinder.Eval(Container.DataItem, "SupplierOnInvoice.Name")%>
                            </td>
                            <td>
                                <%#DataBinder.Eval(Container.DataItem, "Payee.Name")%>
                           </td>                               
                           <td>
                                <%#DataBinder.Eval(Container.DataItem, "InvoiceAmount")%>
                           </td>                                                       
                            
                        </tr>            
                        </ItemTemplate>                            
                    </asp:Repeater>

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