Saving via Reader-Enabled plugins - An unimplemented or obsolete function was called

Question

Hello,
I have recently Reader-Enabled my plugin, and am getting an issue using PDDocSave; getting "An unimplemented or obsolete function was called".
Testing in X and XI has the same response.
Reading Re: Acrobat Reader X and PDDocSave it seems that it is a bug in X, but I would have assumed something like this would be fixed by XI.
The below code has it saving the file where it is, but I am also calling the function with the PDSaveCopy flag to a different location and get the same result.
AVDoc avDoc = AVAppGetActiveDoc();
PDDoc pdDoc = AVDocGetPDDoc(avDoc);
ASFile asFile = PDDocGetFile(pdDoc);
ASFileSys asFileSys = ASFileGetFileSys(asFile);
ASPathName asPath = ASFileAcquirePathName(asFile);
PDDocSave(pdDoc, PDSaveFull | PDSaveLinearized | PDSaveCollectGarbage, asPath, asFileSys, NULL, NULL);
I thought perhaps the save flags may have been the issue, but eliminating one-by-one had no change.
I don’t have the /READER_PLUGIN conditional on, if I do I cannot compile at all - Re: How can I save current document in my Reader plugin? 
Thanks for your help. Thought I'd ask here before putting in a support request.

Answer 1

One thing I notice in the docs for PDDocSave:
IMPLEMENTATION LIMIT: the current implementation can
only save a file to the same fileSys for the doc's current backing
file, thus fileSys MUST BE NULL, otherwise the error XXX is raised.
This is also reiterated in the other headers:
@param fileSys The file system. If it is NULL, uses the fileSys
of the document's current backing file. Files can only be
saved to the same file system. fileSys must be either NULL
or the default file system obtained with ASGetDefaultFileSys(),
otherwise an error is raised.