Tried to make a type of C union, but failed

marshal.fs

open System
open System.Runtime.InteropServices

(* C code
union S {
    uint32_t u32[4];
    uint64_t u64[2];
};
*)

[<Struct>]
[<StructLayout(LayoutKind.Explicit)>]
type S =
   [<MarshalAs(UnmanagedType.ByValArray, SizeConst = 4, ArraySubType = UnmanagedType.U4)>]
   [<FieldOffset(0)>]
   val u32 : uint32 []
   [<MarshalAs(UnmanagedType.ByValArray, SizeConst = 2, ArraySubType = UnmanagedType.U8)>]
   [<FieldOffset(0)>]
   val u64 : uint64 []

[<EntryPoint>]
let main args =
   let ptr = Marshal.AllocHGlobal (Marshal.SizeOf (typeof<S>))
   try
      Marshal.Copy ([| 1L; 2L |], 0, ptr, 2)
      let s = Marshal.PtrToStructure (ptr, typeof<S>) :?> S
      printfn "%A" s.u32  // expected [|1u; 0u; 2u; 0u|], actual [|1UL; 2UL|]
      printfn "%A" s.u64  // expected [|1UL; 2UL|], actual [|1UL; 2UL|]
   finally
      Marshal.FreeHGlobal (ptr)
   0