II7: ISAPI Wildcard Extension generates 500 error (0x8007007f)

I've written an ISAPI extension using Visual Studio 2012 on windows 7 that is a very simple passthrough wildcard extension. When compiled as 32 bit, and with the app pool set to allow 32 bit applications the filter works well. But when I compile as 64 bit (and change the pool to NOT allow 32 bit) I get a 500 error with the code 0x8007007f. 64 bit depends shows all green. There are no errors in the event log. I tried moving the DLL to C:\Windows\System32 but am getting the same error. Any help would be greatly appreciated.

Thanks

Marc

The reason might have been that the 64 bit DLL did not export the functions HttpExtensionProc and GetExtensionVersion, which the error name hints at:

Logfile:
Notification
EXECUTE_REQUEST_HANDLER 
ErrorCode
The specified procedure could not be found.
 (0x8007007f) 

I had missed adding the .def file to the linker options in my project.