diff --git a/src/Plugin/DisplayBuilder/StandardDisplayBuilder.php b/src/Plugin/DisplayBuilder/StandardDisplayBuilder.php
index 5ba0952..d24f966 100644
--- a/src/Plugin/DisplayBuilder/StandardDisplayBuilder.php
+++ b/src/Plugin/DisplayBuilder/StandardDisplayBuilder.php
@@ -2,6 +2,7 @@
 
 namespace Drupal\panels\Plugin\DisplayBuilder;
 
+use Drupal\Component\Render\FormattableMarkup;
 use Drupal\Component\Utility\Html;
 use Drupal\Core\Cache\CacheableMetadata;
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
@@ -117,33 +118,36 @@ class StandardDisplayBuilder extends DisplayBuilderBase implements PluginWizardI
             '#derivative_plugin_id' => $block->getDerivativeId(),
           ];
 
-          // Build the block and bubble its attributes up if possible. This
-          // allows modules like Quickedit to function.
-          // See \Drupal\block\BlockViewBuilder::preRender() for reference.
           $content = $block->build();
-          if ($content !== NULL && !Element::isEmpty($content)) {
+          if (!is_array($content)) {
+            throw new \LogicException(new FormattableMarkup('Block %block build method has to return an array.', ['%block' => $block_id]));
+          }
+
+          // If the block is not empty, build the block and bubble its
+          // attributes up if possible. This allows modules like Quickedit to
+          // function.
+          // If the block is empty, instead of trying to render the block
+          // correctly return just #cache, so that the render system knows the
+          // reasons (cache contexts & tags) why this block is empty.
+          // @see \Drupal\block\BlockViewBuilder::preRender()
+          if (!Element::isEmpty($content)) {
             foreach (['#attributes', '#contextual_links'] as $property) {
               if (isset($content[$property])) {
                 $block_render_array[$property] += $content[$property];
                 unset($content[$property]);
               }
             }
+            $block_render_array['content'] = $content;
           }
-
-          // If the block is empty, instead of trying to render the block
-          // correctly return just #cache, so that the render system knows the
-          // reasons (cache contexts & tags) why this block is empty.
-          if (Element::isEmpty($content)) {
+          else {
             $block_render_array = [];
             $cacheable_metadata = CacheableMetadata::createFromObject($block_render_array);
             $cacheable_metadata->applyTo($block_render_array);
             if (isset($content['#cache'])) {
-              $block_render_array += $content['#cache'];
+              $block_render_array['#cache'] = $content['#cache'];
             }
           }
 
-          $block_render_array['content'] = $content;
-
           $build[$region][$block_id] = $block_render_array;
         }
       }