Error A pointer parameter does not point to accessible memory

Question

i'm build apps using CEN XFS Standard, then i got this error WFS_ERR_INVALID_POINTER in CEN XFS document it was meaning like this A pointer parameter does not point to accessible memory. but i dont know what this error,,

my WFSExecute like this :

WFSExecute(lphService, 1401, cardDispense, 0, lppResult);

and my lppResult like this :

    [StructLayout(LayoutKind.Explicit), Serializable]
    public struct WFSResult
    {
        [FieldOffset(0), MarshalAs(UnmanagedType.U4)]
        public uint RequestID;

        [FieldOffset(4), MarshalAs(UnmanagedType.U2)]
        public ushort hService;

        [FieldOffset(6), MarshalAs(UnmanagedType.Struct, SizeConst = 16)]
        public Timestamp tsTimestamp;

        [FieldOffset(22), MarshalAs(UnmanagedType.U4)]
        public int hResult;

        [FieldOffset(26), MarshalAs(UnmanagedType.U4)]
        public UInt32 dwCommandCode;

        [FieldOffset(26), MarshalAs(UnmanagedType.U4)]
        public UInt32 dwEventID;

        [FieldOffset(30), MarshalAs(UnmanagedType.U4)]
        public Int32 lpBuffer;
    }