error converting list to tuple

[Stefan Seefeld]

Jeff Epler wrote:
 > You can't use a list in all the places you can use a tuple.
 > The most frequently encountered difference is that you can't use lists
 > as keys in dictionaries.

indeed, which makes the error all the more puzzling: somewhere a
list is used as a key, or so it appears. But why do I get this exception
while calling 'list(my_tuple)' ?

Regards,
		Stefan