Fatal error: Default value for parameters with a class type hint can only be NULL in PHP

I am running PHP 5.6 & Laravel 5.6 version project. When I clone it from github repo & run "composer install or composer update". It throws me following error.

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

Is it the problem with dependencies?

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