Gridview1_RowCommand Causes HTTP Error 400

I am in the process of converting an old version of an asp.net application to newer controls. I am trying to use a gridview and I have it populated from the SQL Server. I have several asp:button fields for "Edit" and "Delete". When I click either of the buttons the code behind works but when it completes the procedure the postback causes an HTTP Error 400 - Bad Request.

I commented out the lines...I know this is fired with a post back but not sure why the page does not return to it previous display.

               <asp:Panel id ='referenceLevelLabels' runat="server" CssClass="panel">
                    <asp:GridView ID="referenceLevelLabel" runat="server" AutoGenerateColumns="False" CssClass="gridview">

                        <AlternatingRowStyle BackColor="LightSteelBlue" />
                        <Columns>
                            <asp:TemplateField HeaderText="Action">
                                <itemtemplate>
                                    <asp:Button ID="refEdit" Text="Edit" CommandName="Edit" runat="server" CommandArgument="<%# CType(Container,GridViewRow).RowIndex %>" />
                                    <asp:Button ID="refDelete" Text="Delete" CommandName="Delete" runat="server" CommandArgument="<%# CType(Container,GridViewRow).RowIndex %>" />
                                </itemtemplate>
                            </asp:TemplateField>
                            <asp:BoundField DataField="referenceLevelLabel" HeaderText="Label"  SortExpression="referenceLevelLabel" >
                                <ItemStyle BackColor="BlanchedAlmond" />
                            </asp:BoundField>
                            <asp:BoundField DataField="referenceLevelStatus" HeaderText="Status" />
                            <asp:BoundField DataField="isDefaultLabel" HeaderText="Is Default" />
                        </Columns>
                    </asp:GridView>                    
                </asp:Panel>

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    Private Sub referenceLevelLabel_RowCommand(sender As Object, e As System.Web.UI.WebControls.GridViewCommandEventArgs) Handles referenceLevelLabel.RowCommand
        ' If multiple ButtonField column fields are used, use the
        ' CommandName property to determine which button was clicked.
        If e.CommandName = "Edit" Then

            '' Convert the row index stored in the CommandArgument
            '' property to an Integer.
            'Dim index As Integer = Convert.ToInt32(e.CommandArgument)

            '' Get the last name of the selected author from the appropriate
            '' cell in the GridView control.
            'Dim selectedRow As GridViewRow = referenceLevelLabel.Rows(index)
            'Dim labelCell As TableCell = selectedRow.Cells(referenceColumns.label)
            'Dim reference As String = labelCell.Text

            ''lblmessage.Text = "You selected " & reference & "."
            ''lblmessage.Visible = True

        End If
    End Sub

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