Yii is null MySQL Expression

boolFilter.php

<?php

    protected function boolFilter($query, $field)
    {
        if ($this->$field === '0') {
            return $query->andFilterWhere(['=', 'deposit', $this->$field])
                ->orFilterWhere(['is', $field, new \yii\db\Expression('null')]);
        }

        return $query->orFilterWhere(['=', $field, $this->$field]);
    }

Yii2 is null MySQL Expression.php

<?php

// new \yii\db\Expression('ISNULL(foo)'),
if ($this->foo === '0') {
    $query->andFilterWhere(['is', 'foo', new \yii\db\Expression('null')])
        ->orFilterWhere(['=', 'foo', $this->foo]);
}