what I mean is that you should use a comparison to ``null`` for the case of no provided sequence name, rather than using a string comparison for the case of a provided sequence name.

If you have a sequence name which is a numeric string (not sure if that is allowed on Informix as I never used it), it should still trigger the logic for named sequences even if you pass it as a number rather than a string (while ``is_string`` would return false).