phpunit array_merge(): Expected parameter 1 to be an array, null given code example

Example: array_merge(): Expected parameter 1 to be an array, null given at vendor/phpunit/phpunit/phpunit:61

// PHPUnit uses the constructor for initialization of the base TestCase.
// You shouldn't use the constructor, because it's used by phpunit and any
// change to the signature etc can break things. You can use the special
// setUp and setUpBeforeClass methods which phpunit will call for you.

class XTest extends TestCase
{
    function static setUpBeforeClass()
    { 
       // Called once just like normal constructor
       // You can create database connections here etc
    }

    function setUp()
    {
      //Initialize the test case
      //Called for every defined test
    }

    function testX()
    {
        $this->assertTrue(true);
    }

    // Clean up the test case, called for every defined test
    public function tearDown() { }

    // Clean up the whole test class
    public static function tearDownAfterClass() { }
}