PHP Fatal error: Default value for parameters with a class type hint can only be NULL in /usr/share/php/Symfony/Component/

I cloned new project from github which has following dependencies:

"require": {
    "php": ">=5.6.4",
    "backpack/base": "^0.7.16",
    "backpack/crud": "^3.2",
    "backpack/permissionmanager": "^2.1",
    "barryvdh/laravel-cors": "^0.9.2",
    "laravel/framework": "5.4.*",
    "laravel/passport": "^2.0",
    "laravel/tinker": "~1.0",
    "league/fractal": "^0.16.0"
},

When I am running composer update or any other command, it shows the error as follows:

PHP Fatal error: Default value for parameters with a class type hint can only be NULL in /usr/share/php/Symfony/Component/Console/Application.php on line 85

Fatal error: Default value for parameters with a class type hint can only be NULL in /usr/share/php/Symfony/Component/Console/Application.php on line 85

I am currently using PHP 5.6. What could be the problem?

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