PDO::FETCH_KEY_PAIR errors when selecting not exactly 2 columns

[staabm]

    public function unknownWhereColumn(PDO $pdo)
    {
        $pdo->query('SELECT * FROM ada', PDO::FETCH_KEY_PAIR);
    }

SQLSTATE[HY000]: General error: PDO::FETCH_KEY_PAIR fetch mode requires the result set to contain exactly 2 columns.