MFStartup fails with MF_E_BAD_STARTUP_VERSION even with Platform Update Supplement for Windows Vista installed

I'm attempting to use certain interfaces, such as IMFSinkWriter, on Vista, but I'm having the same problem as this guy:

http://stackoverflow.com/questions/7403687/how-to-use-mediafoundation-imfsourcereader-on-vista-sp2-with-platform-update

I have verified that the Platform Update Supplement for Windows Vista is installed on the machine. I'm building against the Windows SDK 7.1 and am setting the target platform to WINVER=0x0601 since I'm using an interface that's otherwise not available on Vista.

However, MFStartup() fails with MF_E_BAD_STARTUP_VERSION even though the Platform Update Supplement for Windows Vista is supposed to make this work.

Help!