I have the following vertex declration:

struct MESHVERTInstanced
{
    float x, y, z;      // Position
    float nx, ny, nz;   // Normal
    float tu, tv;       // Texcoord
    float idx;          // index of the vertex!
    float tanx, tany, tanz;   // The tangent

    const static D3DVERTEXELEMENT9 Decl[6];
    static IDirect3DVertexDeclaration9* meshvertinstdecl;
};

And I declare it as such:

const D3DVERTEXELEMENT9 MESHVERTInstanced::Decl[] =
{
    { 0, 0,  D3DDECLTYPE_FLOAT3, D3DDECLMETHOD_DEFAULT, D3DDECLUSAGE_POSITION, 0 },
    { 0, 12, D3DDECLTYPE_FLOAT3, D3DDECLMETHOD_DEFAULT, D3DDECLUSAGE_NORMAL,   0 },
    { 0, 24, D3DDECLTYPE_FLOAT2, D3DDECLMETHOD_DEFAULT, D3DDECLUSAGE_TEXCOORD, 0 },
    { 0, 32, D3DDECLTYPE_FLOAT1, D3DDECLMETHOD_DEFAULT, D3DDECLUSAGE_TEXCOORD, 1 },
    { 0, 36, D3DDECLTYPE_FLOAT3, D3DDECLMETHOD_DEFAULT, D3DDECLUSAGE_TANGENT, 0 },
    D3DDECL_END()
};

What I try to do next is copy an ID3DXMesh into another one with the new vertex declaration as such:

model->CloneMesh( model->GetOptions(), MESHVERTInstanced::Decl,
            gd3dDevice, &pTempMesh );

When I try to get the FVF size of pTempMesh (D3DXGetFVFVertexSize(pTempMesh->GetFVF())) I get '0' though the size should be 48.

The whole thing is fine if I don't have the last declaration, '{ 0, 36, D3DDECLTYPE_FLOAT3, D3DDECLMETHOD_DEFAULT, D3DDECLUSAGE_TANGENT, 0 },' in it and the CloneMesh function does not return a FAIL. I've also tried using different declarations such as D3DDECLUSAGE_TEXCOORD and that has worked fine, returning a size of 48. Is there something specific about D3DDECLUSAGE_TANGENT I don't know?

I'm at a complete loss as to why this isn't working...