What would be useful is a test case to see exactly what you are doing. However the JDBC API setNull(parameterIndex, type) does actually require a type.
We implemented the 'unspecified' solution to allow us to use ENUMs, but when we try and enter a NULL value through JDBC to an ENUM field, we get the an error about not being able to insert a varchar into an ENUM …
but NULL isn't varchar or anything else … so why are we getting that sort of message, and is there some way to fix?