Invalid ISBN and RDA

Questions:
If I have an invalid ISBN on my resource, do I qualify it with (invalid) e.g. 020 !z 1742084343 (invalid)? Is the MARC coding $z (Cancelled/invalid ISBN) enough to indicate an invalid ISBN? RDA 2.19.1.5

Answer:
The  correct MARC subfield coding should always be used, and is all that is required to satisfy the RDA requirement to indicate that the ISBN is invalid.  However, some areas are in the practice of also including a qualifier, and it is ok for this practice to continue if considered useful.
(From: NLA RDA FAQ)