not able to leave module pool screen

Hi Experts ,

                  i created a zscreen in function module  using statement 'CALL SCREN ' statment .

                  but i am unable to come back to function module  when i press 'OK' button    even though i used

                  'Leave to screen 0'  in PAI of screen.

                code in function module is

EXPORT P_MAIL TO MEMORY ID 'ZMAIL' .

call SCREEN 101  STARTING AT 40 4.

IMPORT V_MAIL FROM MEMORY ID 'VMAIL'.

   and code in PAI of SCREEN is .

MODULE USER_COMMAND_0101 INPUT.

if OK_code = 'MAIL' .

  EXPORT v_mail to MEMORY id 'VMAIL'.

  leave to SCREEN 0.

  ELSEIF OK_CODE = 'CANCEL'.

  LEAVE TO CURRENT TRANSACTION.

  ENDIF.

END MODULE.

any solutions .