I am trying to setup a simple test that tests some breadcrumb altering code I have introduced only for a specific content type.

Here is the test, still just barebones:

namespace Drupal\mymodule_breadcrumbs\Tests;

use Drupal\simpletest\WebTestBase;

/**
 * Class ArticleBreadcrumbTest
 * @group mymodule_breadcrumbs
 */
class ArticleBreadcrumbTest extends WebTestBase {
  protected $node;
  private $account;

  /**
   * Modules to install.
   * @var array
   */
  public static $modules = ['node', 'mymodule_breadcrumbs', 'menu_breadcrumb'];

  /**
   * Set up our test.
   */
  public function setUp() {
    parent::setUp();

    // create admin user
    $this->account = $this->drupalCreateUser(['administer site configuration', 'administer content']);

    // create article node
    $this->node = $this->drupalCreateNode(array(
      'title' => t('Test Article'),
      'type' => 'article',
      'status' => 1,
    ));
  }

  public function testNodeTitleIsInBreadcrumb() {
    $this->drupalGet('node/' . $this->node->id());
    $this->assertResponse(200);
    #$this->assertText('Home > ' . $this->node->getTitle(), 'Breadcrumb contains the node title.');
    // get page
    // test that breadcrumb is correct per node title
    // change node title
    // save node
    // visit node, check that breadcrumb is updated
  }
}

However, I am getting this error and not sure what to make of it:

The website encountered an unexpected error. Please try again later.

InvalidArgumentException: Invalid database prefix: in Drupal\Core\Test\TestDatabase->__construct() (line 81 of core/lib/Drupal/Core/Test/TestDatabase.php).
simpletest_log_read(NULL, , ) (Line: 476)
_simpletest_batch_finished(, Array, Array, '1 sec')
call_user_func_array('_simpletest_batch_finished', Array) (Line: 414)
_batch_finished() (Line: 81)
_batch_page(Object) (Line: 55)
Drupal\system\Controller\BatchController->batchPage(Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 574)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
call_user_func_array(Object, Array) (Line: 144)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 64)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 99)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 78)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 50)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 652)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

It's a very basic test, why would it cause an error like that?