Developer 2000: can't change a role

  I use Developer/2000 Forms 5.0.6.19.0 with OracleServer 7.2   While the code

     begin
        DBMS_SESSION.SET_ROLE('a_role identified by passwd');
     end;

  is being executed the error "ORA-00600" is occurred.   But the same code is being executed from SqlPlus correctly.   Can you give me any advice? What is the way out from the situation?

Thank you very much for your help and kind attention.

--
Best regards,
 Mike Kogan