Weirdness in PHPSpec?

BuildingId.php

<?php
namespace IronSteel\Construction;

use IronSteel\Library\Uuid;

class BuildingId extends Uuid
{

}

BuildingIdSpec.php

<?php
namespace spec\IronSteel\Construction;

use IronSteel\Construction\BuildingId;
use PhpSpec\ObjectBehavior;
use Prophecy\Argument;

class BuildingIdSpec extends ObjectBehavior
{
    function it_is_initializable()
    {
        $this->beConstructedWith('abcdef');
        $this->shouldHaveType(BuildingId::class);
    }

    function it_should_generate_itself()
    {
        $this->beConstructedThrough('generate');
        $this->shouldHaveType(BuildingId::class);
        $this->toString()->shouldBeString(); // This is the problem call/assertion
    }
}

phpspec output.cli

IronSteel/Construction/BuildingId
  16  - it should generate itself
      method [array:2] not found.

                        75%                                25%         4
2 specs
4 examples (3 passed, 1 broken)
10ms


  Do you want me to create `Ramsey\Uuid\Uuid::isString()` for you? [Y/n]

Uuid.php

<?php
namespace IronSteel\Library;

use Ramsey\Uuid\Uuid as Ruuid;

/**
 * Base class for all UUIDs on the system.
 */
abstract class Uuid
{
    /**
     * Stores the ID.
     *
     * @var string
     */
    private $id;

    /**
     * Constructs a new UUID object.
     *
     * @param string $id
     */
    public function __construct($id)
    {
        $this->id = $id;
    }

    /**
     * Generates a new version 4 UUID object.
     *
     * @return static
     */
    public static function generate()
    {
        return new static(Ruuid::uuid4());
    }

    /**
     * Syntactical sugar for string conversion.
     *
     * @return string
     */
    public function toString()
    {
        return $this->id;
    }

    /**
     * Returns the UUID as a string.
     *
     * @return string
     */
    public function __toString()
    {
        return $this->toString();
    }
}