diff --git a/core/lib/Drupal/Core/Block/BlockManager.php b/core/lib/Drupal/Core/Block/BlockManager.php
index 8f5cd7515d..8230b24142 100644
--- a/core/lib/Drupal/Core/Block/BlockManager.php
+++ b/core/lib/Drupal/Core/Block/BlockManager.php
@@ -3,6 +3,7 @@
 namespace Drupal\Core\Block;
 
 use Drupal\Component\Plugin\Definition\PluginDefinitionInterface;
+use Drupal\Component\Plugin\Factory\DefaultFactory;
 use Drupal\Component\Plugin\FallbackPluginManagerInterface;
 use Drupal\Core\Cache\CacheBackendInterface;
 use Drupal\Core\Extension\ModuleHandlerInterface;
@@ -67,18 +68,10 @@ public function processDefinition(&$definition, $plugin_id) {
     parent::processDefinition($definition, $plugin_id);
     $this->processDefinitionCategory($definition);
 
-    $class = NULL;
-    if ($definition instanceof PluginDefinitionInterface) {
-      $class = $definition->getClass();
-    }
-    elseif (is_array($definition) && isset($definition['class'])) {
-      $class = $definition['class'];
-    }
-    if ($class) {
-      $build_method = new \ReflectionMethod($class, 'build');
-      if (!$build_method->hasReturnType() || $build_method->getReturnType()->getName() !== 'array') {
-        @trigger_error('Declaring ::build() without an array return typehint in ' . $class . ' is deprecated in drupal:9.1.0. Typehinting will be required before drupal:10.0.0. See https://www.drupal.org/node/xxxxxxx', E_USER_DEPRECATED);
-      }
+    $class = DefaultFactory::getPluginClass($plugin_id, $definition);
+    $build_method = new \ReflectionMethod($class, 'build');
+    if (!$build_method->hasReturnType() || $build_method->getReturnType()->getName() !== 'array') {
+      @trigger_error('Declaring ::build() without an array return typehint in ' . $class . ' is deprecated in drupal:9.1.0. Typehinting will be required before drupal:10.0.0. See https://www.drupal.org/node/3164649.', E_USER_DEPRECATED);
     }
   }
 
diff --git a/core/modules/aggregator/src/Plugin/Block/AggregatorFeedBlock.php b/core/modules/aggregator/src/Plugin/Block/AggregatorFeedBlock.php
index ddf59bc53c..3bea4f3929 100644
--- a/core/modules/aggregator/src/Plugin/Block/AggregatorFeedBlock.php
+++ b/core/modules/aggregator/src/Plugin/Block/AggregatorFeedBlock.php
@@ -126,6 +126,7 @@ public function blockSubmit($form, FormStateInterface $form_state) {
    * {@inheritdoc}
    */
   public function build(): array {
+    $build = [];
     // Load the selected feed.
     if ($feed = $this->feedStorage->load($this->configuration['feed'])) {
       $result = $this->itemStorage->getQuery()
@@ -155,9 +156,9 @@ public function build(): array {
           '#url' => $feed->toUrl(),
           '#attributes' => ['title' => $this->t("View this feed's recent news.")],
         ];
-        return $build;
       }
     }
+    return $build;
   }
 
   /**
diff --git a/core/modules/system/src/Plugin/Block/SystemMainBlock.php b/core/modules/system/src/Plugin/Block/SystemMainBlock.php
index 402474143c..869b21b3bf 100644
--- a/core/modules/system/src/Plugin/Block/SystemMainBlock.php
+++ b/core/modules/system/src/Plugin/Block/SystemMainBlock.php
@@ -4,6 +4,7 @@
 
 use Drupal\Core\Block\BlockBase;
 use Drupal\Core\Block\MainContentBlockPluginInterface;
+use Drupal\Core\Logger\LoggerChannelTrait;
 
 /**
  * Provides a 'Main page content' block.
@@ -18,6 +19,8 @@
  */
 class SystemMainBlock extends BlockBase implements MainContentBlockPluginInterface {
 
+  use LoggerChannelTrait;
+
   /**
    * The render array representing the main page content.
    *
@@ -36,6 +39,10 @@ public function setMainContent(array $main_content) {
    * {@inheritdoc}
    */
   public function build(): array {
+    if (!is_array($this->mainContent)) {
+      $this->getLogger('system')->error('The system_main_block was placed but ::setMainContent() was not called.');
+      return [];
+    }
     return $this->mainContent;
   }
 
diff --git a/core/tests/Drupal/Tests/Core/Block/BlockManagerTest.php b/core/tests/Drupal/Tests/Core/Block/BlockManagerTest.php
index 824cc19900..87728c64ca 100644
--- a/core/tests/Drupal/Tests/Core/Block/BlockManagerTest.php
+++ b/core/tests/Drupal/Tests/Core/Block/BlockManagerTest.php
@@ -4,9 +4,13 @@
 
 use Drupal\Component\Plugin\Discovery\DiscoveryInterface;
 use Drupal\Core\Block\BlockManager;
+use Drupal\Core\Block\BlockPluginInterface;
+use Drupal\Core\Block\BlockPluginTrait;
 use Drupal\Core\Block\Plugin\Block\Broken;
+use Drupal\Core\Cache\CacheableDependencyTrait;
 use Drupal\Core\Cache\CacheBackendInterface;
 use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\Plugin\PluginBase;
 use Drupal\Tests\UnitTestCase;
 use Psr\Log\LoggerInterface;
 
@@ -42,11 +46,7 @@ protected function setUp(): void {
     $this->logger = $this->prophesize(LoggerInterface::class);
     $this->blockManager = new BlockManager(new \ArrayObject(), $cache_backend->reveal(), $module_handler->reveal(), $this->logger->reveal());
     $this->blockManager->setStringTranslation($this->getStringTranslationStub());
-
-    $discovery = $this->prophesize(DiscoveryInterface::class);
-    // Specify the 'broken' block, as well as 3 other blocks with admin labels
-    // that are purposefully not in alphabetical order.
-    $discovery->getDefinitions()->willReturn([
+    $this->setDefinitions([
       'broken' => [
         'admin_label' => 'Broken/Missing',
         'category' => 'Block',
@@ -56,16 +56,32 @@ protected function setUp(): void {
       'block1' => [
         'admin_label' => 'Coconut',
         'category' => 'Group 2',
+        'class' => TestBlockManagerBlock::class,
       ],
       'block2' => [
         'admin_label' => 'Apple',
         'category' => 'Group 1',
+        'class' => TestBlockManagerBlock::class,
       ],
       'block3' => [
         'admin_label' => 'Banana',
         'category' => 'Group 2',
+        'class' => TestBlockManagerBlock::class,
       ],
     ]);
+  }
+
+  /**
+   * Sets the definitions the block manager will return.
+   *
+   * @param array $definitions
+   *   An array of plugin definitions.
+   */
+  protected function setDefinitions(array $definitions) {
+    $discovery = $this->prophesize(DiscoveryInterface::class);
+    // Specify the 'broken' block, as well as 3 other blocks with admin labels
+    // that are purposefully not in alphabetical order.
+    $discovery->getDefinitions()->willReturn($definitions);
     // Force the discovery object onto the block manager.
     $property = new \ReflectionProperty(BlockManager::class, 'discovery');
     $property->setAccessible(TRUE);
@@ -107,4 +123,47 @@ public function testHandlePluginNotFound() {
     $this->assertSame('broken', $plugin->getPluginId());
   }
 
+  /**
+   * @group legacy
+   * @expectedDeprecation Declaring ::build() without an array return typehint in Drupal\Tests\Core\Block\TestBlockManagerNoArrayReturnTypeBlock is deprecated in drupal:9.1.0. Typehinting will be required before drupal:10.0.0. See https://www.drupal.org/node/3164649.
+   */
+  public function testBuildNoReturnType() {
+    $this->setDefinitions([
+      'block1' => [
+        'provider' => 'test',
+        'class' => TestBlockManagerNoArrayReturnTypeBlock::class,
+      ],
+    ]);
+    $expected = [];
+    $definitions = $this->blockManager->getDefinitions();
+    $this->assertSame($expected, $definitions);
+  }
+
+}
+
+class TestBlockManagerBlock extends PluginBase implements BlockPluginInterface {
+
+  use BlockPluginTrait;
+  use CacheableDependencyTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function build(): array {
+    return [];
+  }
+
+}
+
+class TestBlockManagerNoArrayReturnTypeBlock extends PluginBase implements BlockPluginInterface {
+
+  use BlockPluginTrait;
+  use CacheableDependencyTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function build() {
+  }
+
 }