IS NOT NULL not working in Yii Active Record

I am trying something like this:

public function search() {

        $criteria = new CDbCriteria;

        $criteria->compare('user_details_id', $this->user_details_id);
        $criteria->compare('user_type_id', $this->user_type_id);
        $criteria->compare('customer_basics_id', $this->customer_basics_id);
        $criteria->compare('user_address_id', $this->user_address_id);
        $criteria->compare('user_city_id', $this->user_city_id);
        $criteria->compare('is_active', $this->is_active);
        $criteria->compare('create_dttm', $this->create_dttm, true);
        $criteria->compare('update_dttm', $this->update_dttm, true);

        // if condition is working
        if (isset($_GET['ulip'])) {
            $criteria->addCondition(
                    "customer_basics_id=" . CustomerBasics::getCustomerBasicsId(Yii::app()->session['user_id']), "AND"
            );
            $criteria->addCondition("user_city_id IS NULL");
        // else condition is working
        } else {
            $criteria->addCondition(
                    "customer_basics_id=" . CustomerBasics::getCustomerBasicsId(Yii::app()->session['user_id']), "AND"
            );
            $criteria->addCondition("user_city_id IS NOT NULL");
        }


    return new CActiveDataProvider($this, array(
        'criteria' => $criteria,
        'pagination' => array(
            'pageSize' => 10,
        ),
    ));
}

Here the issue is if condition is working fine and showing results according to the condition but else part is not working and it returns nothing. I think IS NOT NULL is not working here.

What is the issue ?