Slideshow not working with Master/content page

I am having difficulty getting the Slideshow to work.  Here's my situation: -

I have a master page and content page.  On the content page I have the ToolkitScriptManager, an AnimationExtender which works perfectly, and a SlideShowExtender that doesn't.

The SlideShowExtender is defined: -
    <ajaxToolkit:SlideShowExtender ID="SlideShowExtender1" runat="server" TargetControlID="imgSlideshow"
        SlideShowServiceMethod="GetImages" AutoPlay="true" ImageTitleLabelID="lblClick"
        Loop="true" PlayInterval="1000" ImageDescriptionLabelID="lblImageDescription">
    </ajaxToolkit:SlideShowExtender>

In the CodeBehind: GetImages is defined: -
    <WebMethod()> _
    <ScriptMethod()> _
    Public Shared Function GetImages() As Slide()
        Dim slides As New List(Of Slide)()
        Dim sPath As String = HttpContext.Current.Server.MapPath("~/gdbmstr/Slideshow")
        If sPath.EndsWith("\") Then
            sPath = sPath.Remove(sPath.Length - 1)
        End If
        Dim pathUri As New Uri(sPath, UriKind.Absolute)
        Dim files As String() = Directory.GetFiles(sPath)
        For Each file As String In files
            Dim filePathUri As New Uri(file, UriKind.Absolute)
            slides.Add(New Slide() With { _
              .Name = Path.GetFileNameWithoutExtension(file), _
              .Description = Path.GetFileNameWithoutExtension(file) + " Description.", _
              .ImagePath = pathUri.MakeRelativeUri(filePathUri).ToString() _
            })
        Next
        Return slides.ToArray()
    End Function

If I put a checkpoint on the statement Return slides.ToArray() then I can see that slides contains exactly what I expect.   Continuing to step through the code with F10, immediately on exiting from the function an error is thrown:    Call Stack location: AjaxControlToolkit.DLL!AjaxControlToolkit.Slide.ImagePath.get() line 49
(No source available, so the rest of the debugger output isn't worth copying).

I've tried this, but it didn't make any difference: -
1.  Adding buttons etc so that my code exactly matches the sample
2.  Refreshing my Ajax toolkit by downloading a later version for VS2010 ASP.NET V4.0 (which is what I use)
3.  I've searched this forum for SlideShow and tried any likely looking fix to reported problems

Thank you, Robert Barnes.