Can't update table 'activityinst' in stored function/trigger because it is already used by statement which invoked this stored function/trigger.

Hi,
  i have a trigger in mssql like below

and we migrated it to mysql like this:
CREATE TRIGGER UPDATE_ACTIVITYINST AFTER UPDATE ON ACTIVITYINST FOR EACH ROW
BEGIN
    DECLARE id integer;
    set id= new.ID;
        UPDATE ACTIVITYINST SET STATETIME = GETDATE() WHERE ACTIVITYINST.ID = id;

END

but when i tried to test, i entered a new record to ACTIVITYINST table.
and again i tried to update one that row. iam getting "Can't update table 'activityinst' in stored function/trigger because it is already used by statement which invoked this stored function/trigger."
error.
please let me know where im doing worng and what to do.
please reply soon,it is very urgent
Regards
vijji