CakePHP Unit Testing testAction throws: Undefined index SERVER_PORT

We've got PHPUnit testing setup in our CakePHP 2.6.x application, but everytime the example test is run testAction throws this error:

UsersControllerTest::testLogin
Undefined index: SERVER_PORT

For this unit test:

public function testLogin()
{
    $result = $this->testAction( '/users/login' );

    ...

    debug( $result );
}

Anyone know what has been missed? There isn't any mention of SERVER_PORT in the docs under testing.