directx11 - Marshalling C# Structs into DX11 cbuffers

I'm having some issues with the packing of my structure in C# and passing them through to cbuffers I have registered in HLSL. When I pack my struct in one manner the information seems to be able to pass to the shader:

[StructLayout(LayoutKind.Explicit, Size = 16)]
internal struct TestStruct
{
    [FieldOffset(0)]
    public Vector3 mEyePosition;

    [FieldOffset(12)]
    public int type;
}

This works perfectly when used against this HLSL fragment:

cbuffer PerFrame : register(b0)
{
    Vector3 eyePos;
    int type;
}

float3 GetColour()
{
    float3 returnColour = float(0.0f, 0.0f, 0.0f);


    switch(type)
    {
        case 0:
            returnColour = float3(1.0f, 0.0f, 0.0f);
            break;
        case 1:
            returnColour = float3(0.0f, 1.0f, 0.0f);
            break;
        case 2:

            returnColour = float3(0.0f, 0.0f, 1.0f);
            break;
    }

    return returnColour;
}

However, when I use the following structure definitions...

// Note this is 16 because HLSL packs in 4 float 'chunks'.
// It is also simplified, but still demonstrates the problem.

[StructLayout(Layout.Explicit, Size = 16)] 
internal struct InternalTestStruct
{
    [FieldOffset(0)]
    public int type;
}

[StructLayout(LayoutKind.Explicit, Size = 32)]
internal struct TestStruct
{

    [FieldOffset(0)]
    public Vector3 mEyePosition;

    //Missing 4 bytes here for correct packing.

    [FieldOffset(16)]
    public InternalTestStruct mInternal;
}

... the following HLSL fragment no longer works.

struct InternalType
{
    int type;
}

cbuffer PerFrame : register(b0)
{
    Vector3 eyePos;
    InternalType internalStruct;
}


float3 GetColour()
{
    float3 returnColour = float(0.0f, 0.0f, 0.0f);

    switch(internaltype.type)
    {
        case 0:
            returnColour = float3(1.0f, 0.0f, 0.0f);
            break;

        case 1:
            returnColour = float3(0.0f, 1.0f, 0.0f);
            break;
        case 2:
            returnColour = float3(0.0f, 0.0f, 1.0f);
            break;
    }

    return returnColour;
}

Is there a problem with the way I am packing the struct, or is it another issue?

To re-iterate: I can pass a struct in a cbuffer so long as it does not contain a nested struct.