how to make a select options as mandatory

Dear Experts ,

I'm trying to make select options as mandatory in webdynpro. i write code like following

   CALL METHOD wd_this->m_handler->add_selection_field
  EXPORTING
    i_id                         = 'ZTSDATE'

    i_description                = 'Please Select Date range'
*    i_is_auto_description        = ABAP_TRUE
    it_result                    = LT_RANGE_TABLE
    i_obligatory                 = 'X'
    .
On output select options appearing as manadatory means '*' mark is appearing but when i trigger action without giving any value on select options it is going to give an runtime error. can u Explain me how to resolvw it?

Thanking You in adv,

Rajesh