Exception when casting IOleDropTarget pDataObj object - Invalid FORMATETC structure (Exception from HRESULT: 0x80040064 (DV_E_FORMATETC)

I have implemented the IOleDropTarget interface and am also using the IDropTargetHelper interface to show the system icon of the file being dragged.

Part of my DragEnter code looks like this

Public Function OleDragEnter(<[In]> <MarshalAs(UnmanagedType.Interface)> pDataObj As Object, <[In]> <MarshalAs(UnmanagedType.U4)> grfKeyState As Integer, <[In]> <MarshalAs(UnmanagedType.U8)> pt As Long, <[In]> <Out> ByRef pdwEffect As Integer) As Integer Implements IOleDropTarget.OleDragEnter

    Dim x As Integer = CInt(pt And &H7FFFFFFF)
    Dim y As Integer = CInt((pt >> 32) And &H7FFFFFFF)
    Dim winPT As Win32Point
    winPT.x = CInt(pt And &H7FFFFFFF)
    winPT.y = CInt((pt >> 32) And &H7FFFFFFF)

    ddHelper.DragEnter(hwnd, CType(pDataObj, NativeMethods.IDataObject), winPT, 0)
End Function

The cast of the pDataObj to an IDataObject works for every kind of object I can drag except for an Outlook email attachment. An email itself works fine but not an attachment. The error is Invalid FORMATETC structure (Exception from HRESULT: 0x80040064 (DV_E_FORMATETC)

Where do I start to work out what I am doing wrong? What code should I show?

my IDataObject interface looks like this

 <ComImport, Guid("0000010E-0000-0000-C000-000000000046"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)>
    Interface IDataObject
        <PreserveSig>
        Function GetData(
<[In]> ByRef format As FORMATETC, <Out> ByRef medium As STGMEDIUM) As Integer
        <PreserveSig>
        Function GetDataHere(
<[In]> ByRef format As FORMATETC, ByRef medium As STGMEDIUM) As Integer
        <PreserveSig>
        Function QueryGetData(
<[In]> ByRef format As FORMATETC) As Integer
        <PreserveSig>
        Function GetCanonicalFormatEtc(
<[In]> ByRef formatIn As FORMATETC, <Out> ByRef formatOut As FORMATETC) As Integer
        <PreserveSig>
        Function SetData(
<[In]> ByRef formatIn As FORMATETC,
<[In]> ByRef medium As STGMEDIUM,
<MarshalAs(UnmanagedType.Bool)> ByVal release As Boolean) As Integer
        <PreserveSig>
        Function EnumFormatEtc(ByVal direction As ComTypes.DATADIR, <Out> ByRef ppenumFormatEtc As System.Runtime.InteropServices.ComTypes.IEnumFORMATETC) As Integer
        <PreserveSig>
        Function DAdvise(
<[In]> ByRef pFormatetc As FORMATETC, ByVal advf As ComTypes.ADVF, ByVal adviseSink As ComTypes.IAdviseSink, <Out> ByRef connection As Integer) As Integer
        <PreserveSig>
        Function DUnadvise(ByVal connection As Integer) As Integer
        <PreserveSig>
        Function EnumDAdvise(<Out> ByRef enumAdvise As ComTypes.IEnumSTATDATA) As Integer
    End Interface

and FORMATETC structure like this

 <StructLayout(LayoutKind.Sequential)>
    Public NotInheritable Class FORMATETC
        Public cfFormat As UShort
        Public dummy As Short
        Public ptd As IntPtr
        Public dwAspect As Integer
        Public lindex As Integer
        Public tymed As Integer
    End Class

UPDATED ddHelper is an instance of my class cast from the IDropTargetHelper interface.

Private ddHelper As IDropTargetHelper = CType(New DragDropHelper(), IDropTargetHelper)

class and interface look like this

<StructLayout(LayoutKind.Sequential)>
Public Structure Win32Point
    Public x As Integer
    Public y As Integer
End Structure

<ComImport>
<Guid("4657278A-411B-11d2-839A-00C04FD918D0")>
Public Class DragDropHelper
End Class

<ComVisible(True)>
<ComImport>
<Guid("4657278B-411B-11D2-839A-00C04FD918D0")>
<InterfaceType(ComInterfaceType.InterfaceIsIUnknown)>
Interface IDropTargetHelper
    Sub DragEnter(
<[In]> ByVal hwndTarget As IntPtr,
<[In], MarshalAs(UnmanagedType.[Interface])> ByVal dataObject As NativeMethods.IDataObject,
<[In]> ByRef pt As Win32Point,
<[In]> ByVal effect As Integer)
    Sub DragLeave()
    Sub DragOver(
<[In]> ByRef pt As Win32Point,
<[In]> ByVal effect As Integer)
    Sub Drop(
<[In], MarshalAs(UnmanagedType.[Interface])> ByVal dataObject As NativeMethods.IDataObject,
<[In]> ByRef pt As Win32Point,
<[In]> ByVal effect As Integer)
    Sub Show(
<[In]> ByVal show As Boolean)
End Interface

And the error looks like this.