diff --git a/src/Plugin/DisplayBuilder/StandardDisplayBuilder.php b/src/Plugin/DisplayBuilder/StandardDisplayBuilder.php
index 20fbb61..3d26665 100644
--- a/src/Plugin/DisplayBuilder/StandardDisplayBuilder.php
+++ b/src/Plugin/DisplayBuilder/StandardDisplayBuilder.php
@@ -3,6 +3,7 @@
 namespace Drupal\panels\Plugin\DisplayBuilder;
 
 use Drupal\Component\Utility\Html;
+use Drupal\Core\Block\BlockPluginInterface;
 use Drupal\Core\Extension\ModuleHandlerInterface;
 use Drupal\Core\Cache\CacheableMetadata;
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
@@ -102,60 +103,69 @@ class StandardDisplayBuilder extends DisplayBuilderBase implements PluginWizardI
    */
   protected function buildRegions(array $regions, array $contexts) {
     $build = [];
+    /** @var \Drupal\Core\Block\BlockPluginInterface[] $blocks */
     foreach ($regions as $region => $blocks) {
       if (!$blocks) {
         continue;
       }
 
-      $region_name = Html::getClass("block-region-$region");
-      $build[$region]['#prefix'] = '<div class="' . $region_name . '">';
-      $build[$region]['#suffix'] = '</div>';
-
-      /** @var \Drupal\Core\Block\BlockPluginInterface[] $blocks */
       $weight = 0;
+
+      $build[$region]['#prefix'] = '<div class="' . Html::getClass("block-region-$region") . '">';
+      $build[$region]['#suffix'] = '</div>';
       foreach ($blocks as $block_id => $block) {
         if ($block instanceof ContextAwarePluginInterface) {
           $this->contextHandler->applyContextMapping($block, $contexts);
         }
+
         if ($block->access($this->account)) {
-          $block_render_array = [
-            '#theme' => 'block',
-            '#attributes' => [],
-            '#contextual_links' => [],
-            '#weight' => $weight++,
-            '#configuration' => $block->getConfiguration(),
-            '#plugin_id' => $block->getPluginId(),
-            '#base_plugin_id' => $block->getBaseId(),
-            '#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();
           $this->moduleHandler->alter(['block_build', 'block_build_' . $block->getBaseId()], $content, $block);
-          if ($content !== NULL && !Element::isEmpty($content)) {
-            foreach (['#attributes', '#contextual_links'] as $property) {
-              if (isset($content[$property])) {
-                $block_render_array[$property] += $content[$property];
-                unset($content[$property]);
-              }
-            }
+
+          if (!is_array($content)) {
+            throw new \LogicException(sprintf(
+              '%s::build() method has to return an array. Class returning non-array value is: %s',
+              BlockPluginInterface::class,
+              get_class($block)
+            ));
           }
 
           // 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)) {
             $block_render_array = [];
             $cacheable_metadata = CacheableMetadata::createFromObject($block_render_array);
             $cacheable_metadata->applyTo($block_render_array);
             if (isset($content['#cache'])) {
-              $block_render_array['#cache'] += $content['#cache'];
+              $block_render_array['#cache'] = $content['#cache'];
             }
           }
+          else {
+            $block_render_array = [
+              '#theme' => 'block',
+              '#attributes' => [],
+              '#contextual_links' => [],
+              '#weight' => $weight++,
+              '#configuration' => $block->getConfiguration(),
+              '#plugin_id' => $block->getPluginId(),
+              '#base_plugin_id' => $block->getBaseId(),
+              '#derivative_plugin_id' => $block->getDerivativeId(),
+            ];
+
+            // If the block is not empty, build the block and bubble its
+            // attributes up if possible. This allows modules like Quickedit to
+            // function.
+            foreach (['#attributes', '#contextual_links'] as $property) {
+              if (isset($content[$property])) {
+                $block_render_array[$property] += $content[$property];
+                unset($content[$property]);
+              }
+            }
 
-          $block_render_array['content'] = $content;
+            $block_render_array['content'] = $content;
+          }
 
           $this->moduleHandler->alter(['block_view', 'block_view_' . $block->getBaseId()], $block_render_array, $block);
           $build[$region][$block_id] = $block_render_array;