Windows Media Foundation -- IMFActivate::ActivateObject - RPC Error?

I'm having trouble creating a UVC Extension Unit interface using the Windows Media Foundation API (using the Windows 8.1 kit).

I am attempting to open a video source before using functions provided by Ks.h to interface with the video source's respective UVC extension unit.

I enumerate available video sources using

MFEnumDeviceSources()

and selecting the correct device by the enumerated devices' friendly names, which all works fine.

The problem occurs when I attempt to call IMFActivate::ActivateObject() on the selected video source:

HRESULT hr = ppVideoDevices[deviceIndex]->ActivateObject(IID_PPV_ARGS(&pVideoSource));

which results in the following error:

hr = 0x800706d9 : There are no more endpoints available from the endpoint mapper.

I have ensured all of my RPC services are running correctly, but I cannot figure out what might be causing this error. I have attempted to find an existing solution online, but could find nothing.

Has anyone else encountered something similar?