Error can happen when PLI compiler option DEFAULT(LINKAGE(SYSTEM)) is used.
When calling C programs like zUnit AZUTSTRN, PLI compiler documentation indicates to use DEFAULT(LINKAGE((OPTLINK)).
You can use compiler option OPTIONS(ALL) to print all options used in the listing file.
(Disclaimer: This answer is migrated from IBM Developer Answers that is no longer in use. Its original author is Olivier Gauneau.)