VMS Help NCS Routines, NCS$COMPARE, Condition Value Signaled  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    STR$_ILLSTRCLA     Illegal string class. Severe error. The
                       descriptor of string_1 or string_2, or both,
                       contains a class code not supported by the
                       OpenVMS Calling Standard.