SQL & PL/SQL

Good day,

I managed to run the code below (part of my procedure) on the first run but subsequent runs have been failing with an ORA-30926: unable to get a stable set of rows in the source tables. The remedy to this is having to Remove any non-deterministic where clauses and reissue the dml but this being new with merge...any help will be appreciated.

Thanks

begin
MERGE                                                         /*+ APPEND */
        INTO tb1 a
    USING tb2 b
       ON (    a.col1 = b.col1
           AND a.col2 = b.col2)
   WHEN NOT MATCHED
   THEN
      INSERT        (a.col3,
                     a.col4)
      VALUES (val3,
              val4)
   WHEN MATCHED
   THEN
      UPDATE SET a.col3 = a.col3 + 2;

   COMMIT;
   end;