I added a simple check for iterables. This is not very elegant, but
performance is only affected in the case of an exception. Patch and
corresponsing test are attached as "TypeError.patch".

As I pointed out above, the actual error message "must be a sequence" is
also inconsistent with the implementation (and tests) which allows any
kind of iterable. The attached and independent patch
"message_and_docs.patch" changes this to "must be an iterable" and
corrects docs and tests accordingly.