What am I doing wrong?

When it gets to the "MyTextBox.Text = Now" line I get "Object reference not
set to an instance of an object."

PARTIAL HTML:
<asp:DetailsView ID="DetailsView1" runat="server" AutoGenerateRows="False"
       CellPadding="4" DataKeyNames="ID" DataSourceID="adsObitDetails"
ForeColor="#333333"
       GridLines="None" Height="50px" Width="100%">
       <FooterStyle BackColor="#5D7B9D" Font-Bold="True" ForeColor="White"
/>
       <CommandRowStyle BackColor="#E2DED6" Font-Bold="True" />
       <RowStyle BackColor="#F7F6F3" ForeColor="#333333" />
       <FieldHeaderStyle BackColor="#E9ECF1" Font-Bold="True" />
       <PagerStyle BackColor="#284775" ForeColor="White"
HorizontalAlign="Center" />
       <Fields>
 :
           <asp:TemplateField HeaderText="Entered"
SortExpression="InputDateTime">
               <EditItemTemplate>
                   <asp:TextBox ID="txtInputDateTime" runat="server"
                       Text='<%# Bind("InputDateTime") %>'></asp:TextBox>
               </EditItemTemplate>
               <InsertItemTemplate>
                   <asp:TextBox ID="txtInputDateTime" runat="server"
Text='<%# Bind("InputDateTime") %>'></asp:TextBox>
               </InsertItemTemplate>
               <ItemTemplate>
                   <asp:Label ID="Label2" runat="server" Text='<%#
Bind("InputDateTime") %>'></asp:Label>
               </ItemTemplate>
           </asp:TemplateField>
 :
       </Fields>
       <HeaderStyle BackColor="#5D7B9D" Font-Bold="True" ForeColor="White"
/>
       <EditRowStyle BackColor="#999999" />
       <AlternatingRowStyle BackColor="White" ForeColor="#284775" />
   </asp:DetailsView>

CODE:
   Protected Sub DetailsView1_ModeChanging(ByVal sender As Object, ByVal e
As System.Web.UI.WebControls.DetailsViewModeEventArgs) Handles
DetailsView1.ModeChanging
       If e.NewMode = DetailsViewMode.Insert Then
           Dim MyTextBox As TextBox
           MyTextBox =
CType(Me.DetailsView1.FindControl("txtInputDateTime"), TextBox)
           MyTextBox.Text = Now
       End If
   End Sub