Issue with UpdatePanel inside a ModalPopupExtender Panel

I have a ModalPopupExtender that is firing and showing a Panel. Inside that Panel is an UpdatePanel. The Panel has some form fields on it to gather some info, the user clicks a Preview button (trigger button for UpdatePanel) and the code fires off without a hitch. Then when I go to close the ModalPopupExtender, I get the following error:

Microsoft JScript runtime error: Sys.InvalidOperationException: Handler was not added through the Sys.UI.DomEvent.addHandler method.

With a button for Break, Continue, and Ignore.

Here is my ModalPopupExtender Call:

<ajaxToolkit:ModalPopupExtender runat="server" ID="ModalPopupExtender4" TargetControlID="btnSFHEnv" PopupControlID="PnlSFHEnvPopup" DropShadow="true" BackgroundCssClass="ModalPopupCSS" />

And my popup Panel:

<asp:Panel runat="server" ID="PnlSFHEnvPopup" Width="985px" BackColor="#74606c" ><table><tr><td><asp:Label ID="Label1" runat="server" Text="Custom Text 1" ForeColor="White" Font-Bold="true" /></td><td><asp:Label ID="Label3" runat="server" Text="Custom Text 2" ForeColor="White" Font-Bold="true" /></td><td><asp:Label ID="Label4" runat="server" Text="Custom Text 3" ForeColor="White" Font-Bold="true" /></td></tr><tr><td><asp:TextBox runat="server" ID="txtCustomEnv1" Width="150px" /></td><td><asp:TextBox runat="server" ID="txtCustomEnv2" Width="150px" /></td><td><asp:TextBox runat="server" ID="txtCustomEnv3" Width="150px" /></td></tr><tr><td colspan="3"><ajaxToolkit:TextBoxWatermarkExtender runat="server" ID="TextBoxWatermarkExtender1" TargetControlID="txtCustomEnv1" WatermarkText="123 Main St." WatermarkCssClass="Watermark"  /><ajaxToolkit:TextBoxWatermarkExtender runat="server" ID="TextBoxWatermarkExtender2" TargetControlID="txtCustomEnv2" WatermarkText="Suite 1000" WatermarkCssClass="Watermark"  /><ajaxToolkit:TextBoxWatermarkExtender runat="server" ID="TextBoxWatermarkExtender3" TargetControlID="txtCustomEnv3" WatermarkText="City, ST  ZipCode" WatermarkCssClass="Watermark"  /></td></tr><tr><td align="center" colspan="3"><asp:UpdatePanel runat="server" ID="UpdEnvPanel" UpdateMode="Conditional" ><Triggers><asp:AsyncPostBackTrigger ControlID="btnEnvPreview" EventName="Click" /></Triggers><ContentTemplate><table><tr><td><asp:Button runat="server" ID="btnEnvPreview" Text="Preview" 
                            onclick="btnEnvPreview_Click" /></td></tr><tr><td><asp:Image runat="server" ID="ImgSFHEnv_LG" ImageUrl="images/SFHEnv.jpg"/></td></tr></table></ContentTemplate></asp:UpdatePanel>                                                                                                       </td></tr><tr><td colspan="3"><asp:Button runat="server" ID="btnEnvAddToCart" Text="Add to Cart" onclick="btnEnvAddToCart_Click" 
                             /></td></tr>            </table>        </asp:Panel>

I tried stripping out all the excess code and it's not until I add the UpdatePanel that I start getting the error.