diff --git a/core/lib/Drupal/Core/Entity/EntityViewBuilder.php b/core/lib/Drupal/Core/Entity/EntityViewBuilder.php
index 8bfb3a3..070e9c6 100644
--- a/core/lib/Drupal/Core/Entity/EntityViewBuilder.php
+++ b/core/lib/Drupal/Core/Entity/EntityViewBuilder.php
@@ -148,7 +148,6 @@ protected function getBuildDefaults(EntityInterface $entity, $view_mode) {
     $this->moduleHandler()->alter('entity_view_mode', $view_mode, $entity, $context);
 
     $build = array(
-      '#theme' => $this->entityTypeId,
       "#{$this->entityTypeId}" => $entity,
       '#view_mode' => $view_mode,
       // Collect cache defaults for this entity.
@@ -159,6 +158,11 @@ protected function getBuildDefaults(EntityInterface $entity, $view_mode) {
       ),
     );
 
+    // Add the default #theme key if a template exists for it.
+    if (\Drupal::service('theme.registry')->getRuntime()->has($this->entityTypeId)) {
+      $build['#theme'] = $this->entityTypeId;
+    }
+
     // Cache the rendered output if permitted by the view mode and global entity
     // type configuration.
     if ($this->isViewModeCacheable($view_mode) && !$entity->isNew() && $entity->isDefaultRevision() && $this->entityType->isRenderCacheable()) {