Fatal error in Drupal 8 user creation

Visiting http://<example.com>/admin/people/create yields this error:

( ! ) Fatal error: Call to undefined method Drupal\Component\Utility\SafeMarkup::set() in /opt/buildkit/build/d8-master/modules/civicrm/civicrm.module on line 209
Call Stack

1. Time Memory Function Location
   1 0.0000 234656 {main}

   ( ) ../index.php:0
   2 0.0007 578240 Drupal\Core\DrupalKernel->handle( ) ../index.php:19
   3 0.0046 1551040 Stack\StackedHttpKernel->handle( ) ../DrupalKernel.php:637
   4 0.0046 1551152 Drupal\Core\StackMiddleware\NegotiationMiddleware->handle( ) ../StackedHttpKernel.php:23
   5 0.0046 1551624 Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle( ) ../NegotiationMiddleware.php:55
   6 0.0046 1552056 Drupal\page_cache\StackMiddleware\PageCache->handle( ) ../ReverseProxyMiddleware.php:51
   7 0.0048 1559296 Drupal\page_cache\StackMiddleware\PageCache->pass( ) ../PageCache.php:82
   8 0.0048 1559376 Drupal\Core\StackMiddleware\KernelPreHandle->handle( ) ../PageCache.php:103
   9 0.0068 1868720 Drupal\Core\StackMiddleware\Session->handle( ) ../KernelPreHandle.php:53
   10 0.0078 2029064 Symfony\Component\HttpKernel\HttpKernel->handle( ) ../Session.php:62
   11 0.0078 2029920 Symfony\Component\HttpKernel\HttpKernel->handleRaw( ) ../HttpKernel.php:62
   12 0.0240 4830488 call_user_func_array:

   {/opt/buildkit/build/d8-master/vendor/symfony/http-kernel/HttpKernel.php:139}

   ( ) ../HttpKernel.php:139
   13 0.0240 4830688 Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber{closure}( ) ../HttpKernel.php:139
   14 0.0240 4830936 Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext( ) ../EarlyRenderingControllerWrapperSubscriber.php:102
   15 0.0241 4842920 Drupal\Core\Render\Renderer->executeInRenderContext( ) ../EarlyRenderingControllerWrapperSubscriber.php:129
   16 0.0241 4843368 Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber{closure}( ) ../Renderer.php:577
   17 0.0241 4843416 call_user_func_array:

   {/opt/buildkit/build/d8-master/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php:128}

   ( ) ../EarlyRenderingControllerWrapperSubscriber.php:128
   18 0.0241 4844136 Drupal\Core\Controller\FormController->getContentResult( ) ../EarlyRenderingControllerWrapperSubscriber.php:128
   19 0.0392 9032304 Drupal\Core\Form\FormBuilder->buildForm( ) ../FormController.php:79
   20 0.0539 11686112 Drupal\Core\Form\FormBuilder->prepareForm( ) ../FormBuilder.php:277
   21 0.0540 11698472 Drupal\Core\Extension\ModuleHandler->alter( ) ../FormBuilder.php:821
   22 0.0541 11704192 civicrm_form_user_register_form_alter( ) ../ModuleHandler.php:492