Incorrectly aligned or overlapped by a non-object field error

I'm trying to create the following structure:

    [StructLayout(LayoutKind.Explicit, Size=14)]
    public struct Message
    {
        [FieldOffset(0)]
        public ushort X;
        [FieldOffset(2)]
        [MarshalAs(UnmanagedType.ByValArray, SizeConst=5)]
        private ushort[] Y;
        [FieldOffset(12)]
        public ushort Z;
    }

and I get the following error:

Could not load type 'Message' from assembly because it contains an object field at offset 4 that is incorrectly aligned or overlapped by a non-object field.

Does anyone know why this is causing an error?

Note: I can not use Pack because I'm working with the compact framework. Thanks.