A simple problem with validating the form

Hi friends,
I have a drop-down list which has 2 options.when the form is shown to user then I add more options to it by javascript.
by this way I get a validation error:("an illegal selection has been selected")
after searching for problem I found this:
"Elements with options (check boxes, radio buttons, and drop-down selection
fields) are examined to see if the selected value is actually in the original list of options
present when the form was built"(Pro drupal development book)

that is my problem,how can I solve my problem?

thanks and excuse me for english;