Views query options form validation

Hi there,
I have written a query plugin in which there are options form, the form includes a select list that the user should select an item from. Unfortunately, overriding options_validate() to make sure that the user have selected an item is not enough, since the user hit view's save button even without opening query settings form.
Also, I tried overriding views_plugin::validate() but the code does not seem to be executed. Any clues?
Thanks in advance.