MFXVideoCORE_SyncOperation returns MFX_ERR_UNSUPPORTED

I am using Windows.

I have a successful pipeline which is successfully working using multiple sessions and VIDEO memory. I now need to output the resultant frame to SYSTEM memory. I am using the following IOPATTERN

IOPattern = MFX_IOPATTERN_IN_VIDEO_MEMORY | MFX_IOPATTERN_OUT_SYSTEM_MEMORY;

To create a system allocator I use SysMemFrameAllocator.

I create a new session and use MFXJoinSession to join this session to the parent session.

I run MFXVideoVPP_RunFrameVPPAsync which returns ERR_NONE.

The mfxVideoParams for the surface in and surface out are identical.

MFXVideoCORE_SyncOperation returnjs MFX_ERR_UNSUPPORTED. How do I get the raw surface back into SYSTEM memory?

kind regards

Kevin