Direct3D won't reset

I'm having some troubles when alt-tabbing out then back into my game. The Direct3DDevice->Reset() method fails, returning an "invalid parameter" error message. I have no idea what this could be. The device creation goes fine with the parameters, but when I use the same for the reset it fails (I've double chekced, they aren't modified somewhere else along the way). All buffers I use are managed.

Any idea?

These are the parameters I set up Direct3D with:

Code:

---

DeviceInfo.BackBufferWidth = Width;
DeviceInfo.BackBufferHeight = Height;
DeviceInfo.BackBufferFormat = D3DFMT_A8R8G8B8;
DeviceInfo.BackBufferCount = 1;
DeviceInfo.MultiSampleType = D3DMULTISAMPLE_NONE;
DeviceInfo.MultiSampleQuality = 0;
DeviceInfo.SwapEffect = D3DSWAPEFFECT_DISCARD;
DeviceInfo.hDeviceWindow = Window;
DeviceInfo.Windowed = FALSE;
DeviceInfo.EnableAutoDepthStencil = TRUE;
DeviceInfo.AutoDepthStencilFormat = D3DFMT_D16;
DeviceInfo.Flags = 0;
DeviceInfo.FullScreen_RefreshRateInHz = D3DPRESENT_RATE_DEFAULT;
DeviceInfo.PresentationInterval = D3DPRESENT_INTERVAL_DEFAULT;

---

This is the function I call before every rendering session:

Code:

---

BOOL GRAPHICS::BeginRender()
{
        HRESULT Result;

        //Clears the debug information
        BlocksRendered = 0;
        SidesRendered = 0;

        //Checks if the program lost focus
        Result = Direct3DDevice->TestCooperativeLevel();
        if(Result != D3D_OK)
        {
                //If you cannot simply reset, wait until you can
                if(Result == D3DERR_DEVICELOST)
                {
                        return FALSE;
                }

                //Resets the device
                if(FAILED(Direct3DDevice->Reset(&DeviceInfo)))
                {
                        return FALSE;
                }

                //Re-sets the render states
                SetupRenderStates();
        }

        //Clears the back buffer and Z-buffer
        if(FAILED(Direct3DDevice->Clear(0, NULL, D3DCLEAR_TARGET | D3DCLEAR_ZBUFFER,
                                                                        BackgroundColor, 1.0, 0)))
        {
                return FALSE;
        }

        //Begins rendering
        if(FAILED(Direct3DDevice->BeginScene()))
        {
                return FALSE;
        }

        return TRUE;
}

---

No one knows? :/