Using path mapping modifier fails when given key is an integer

[Romain Canon]

\CuyZ\Valinor\Mapper\Source\Source::array([0 => 'foo'])
    ->map([
        '0' => 'some_key'
    ]);

PHP Fatal error:  Uncaught TypeError: explode(): Argument #2 ($string) must be of type string, int given in /…/src/Mapper/Source/Modifier/PathMapping.php:73