PL/SQL table next function, Oracle 7 vs Oracle 8

Is there any reason why the following statement would work just fine in Oracle 7, but appears to hang in Oracle 8:

 rec_id := lock_tab.next(rec_id);

Appropriate definitions:

  type lock_id_rectype is record (
     select_ckp natural,   -- select checkpoint
     modify_ckp natural,   -- modification checkpoint
     delete_ckp natural,   -- create/delete checkpoint
     reserve_ckp natural); -- check out checkpoint
  

  type lock_id_tabtype is table of lock_id_rectype     index by binary_integer;

  lock_tab lock_id_tabtype;

Thanks. I'm currently stumped.

  Francis

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