Why detailsview in a modalpopup doesn't display the correct selected gridview row?

I have a gridview and I want to be able to select a row and display it on a detailsview displayed as a modalpopup. However, the details view only shows the first row of the gridview, but if I make the detailsview not to display in a modalpopup, it shows the correct gridview row selection. What am I doing wrong?

<asp:GridView ID="GridView5" runat="server" AllowPaging="True"
  AllowSorting="True" AutoGenerateColumns="False"
    DataKeyNames="WorkshopName"
        DataSourceID="SqlDataSource8" SelectedIndex="0"Width="500px" PageSize="5"
            onselectedindexchanged="GridView5_SelectedIndexChanged"><Columns><asp:TemplateField ShowHeader="False"><ItemTemplate><asp:ImageButton ID="ImageButton1" runat="server" CausesValidation="False"
    CommandName="Select" ImageUrl="~/Images/Select.png" Text="Select" /><asp:ModalPopupExtender ID="ImageButton1_ModalPopupExtender" runat="server"
DynamicServicePath="" Enabled="True" PopupControlID="DetailsView5"
TargetControlID="ImageButton1"></asp:ModalPopupExtender></ItemTemplate></asp:TemplateField><asp:DetailsView ID="DetailsView5" runat="server" AutoGenerateRows="False"
DataKeyNames="WorkshopName,WorkshopBeginingDate,WorkshopLocation"
DataSourceID="SqlDataSource9" Height="50px" Width="330px" GridLines="None"
DefaultMode="Edit" BackColor="#CCCCCC" onitemupdated="DetailsView5_ItemUpdated">


protected void DetailsView5_ItemUpdated(object sender, DetailsViewUpdatedEventArgs e)
    {
if (e.Exception != null)
    {
lblerror3.Text = "A database error has ocurred. " + "Message: " + e.Exception.Message;
e.ExceptionHandled = true;
e.KeepInEditMode = true;
    }
else if (e.AffectedRows == 0)
lblerror3.Text = "Another user might have updated that workshop. " + "Please try again.";
GridView5.DataBind();
    }
protected void GridView5_SelectedIndexChanged(object sender, EventArgs e)
    {
DetailsView5.PageIndex = GridView5.SelectedIndex;
    }