Objects/tupleobject.c:125: bad argument to internal function

[Thomas Levine <_ thomaslevine com>]

I'm writing functions in Python, and here's what happens.

1. Write '=major_third_interval("C4")', or another of my
   functions, in a few cells.
2. Everything works as I expect.
3. I save the file and close Gnumeric.
4. I start Gnumeric and open the file.
5. Everything works as I expect except for the top-left-most
   cell function call. The following error is rendered in the
   cell, and nothing is printed at the console.

    Python exception (<type 'exceptions.SystemError'>: Objects/tupleobject.c:125: bad argument to internal 
function)

The plugin is called "sheetmusic", and here is the code.

    import libsheetmusic.main
    sheetmusic_functions = libsheetmusic.main.functions()

The following links will all get you to the libsheetmusic package.
https://pypi.python.org/pypi/sheetmusic
http://dadawarehouse.thomaslevine.com/small/sheetmusic/
https://github.com/tlevine/sheetmusic

Any ideas of what's going on?

Thanks

Tom