diff -u b/core/modules/block/lib/Drupal/block/BlockViewBuilder.php b/core/modules/block/lib/Drupal/block/BlockViewBuilder.php
--- b/core/modules/block/lib/Drupal/block/BlockViewBuilder.php
+++ b/core/modules/block/lib/Drupal/block/BlockViewBuilder.php
@@ -72,7 +72,7 @@
         array('content' => TRUE),
         $this->getCacheTag(), // Block view builder cache tag.
         $entity->getCacheTag(), // Block entity cache tag.
-        array('theme' => array($entity->id())), // Block entity theme.
+        array('theme' => $entity->get('theme')), // Block entity theme.
         $plugin->getCacheTags(), // Block plugin cache tags.
       ));