ASP.NET Gridview with linkbutton

I have a gridview with linkbutton in it. But I'm hving trouble using the datakey value in my codebehind. I get an error "System.FormatException: Input string was not in a correct format."

Any help on accessing the datakeynames value?

<asp:GridView ID="GridTree" runat="server" DataSourceId="sdsTree" AutoGenerateColumns="False" BorderWidth="0" ShowHeader="false" DataKeyNames="xId">
                            <RowStyle BackColor="White" Height="14" VerticalAlign="Middle" Font-Size="X-Small" />
                            <AlternatingRowStyle BackColor="#B5B4B2" Height="14" VerticalAlign="Middle" Font-Size="X-Small" />
                            <Columns>
                                <asp:TemplateField ItemStyle-Width="173">
                                    <ItemTemplate>
                                        <asp:LinkButton ID="linkTree" CommandName="NodeSelected" runat="server" style="text-decoration:none; color:#000000; "><%# Eval("xLabel") %></asp:LinkButton>
                                    </ItemTemplate>
                                </asp:TemplateField>
                            </Columns>            
                        </asp:GridView>
 
 
    Protected Sub GridTree_RowCommand(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewCommandEventArgs) Handles GridTree.RowCommand
        Dim yNode As Integer
        yNode = Me.GridTree.DataKeys(Integer.Parse(e.CommandArgument.ToString())).Value
        Dim yChunk As Integer
        yChunk = xGetChunkId(yNode)
        Select Case e.CommandName
            Case "NodeSelected"
                If yChunk Then
                    ViewState("vsTreeParent") = Me.GridTree.DataKeys(Integer.Parse(e.CommandArgument.ToString())).Value
                    Call xShowTreeNodes(ViewState("vsTreeParent"))
                Else
                    xShowChunk(yChunk)
                End If
        End Select
    End Sub

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