FilePutObject issues

[Vax]

Am I missing something? I try to save a dataset object as a binary file object, but I keep getting an I/O exception about the type being invalid. At one point I had something like this working, and now its not and I have no idea what I did:

iFile = FreeFile()
FileOpen(iFile, .FileName, OpenMode.Binary)
FilePutObject(iFile, myDataset)
FileClose(iFile)

any ideas?

-Vax