diff --git a/modules/opd_daily_emails/src/DailyEmails.php b/modules/opd_daily_emails/src/DailyEmails.php
index 0041c7ec5..f2bde6c8d 100644
--- a/modules/opd_daily_emails/src/DailyEmails.php
+++ b/modules/opd_daily_emails/src/DailyEmails.php
@@ -5,6 +5,7 @@ declare(strict_types=1);
 namespace Drupal\opd_daily_emails;
 
 use Drupal\node\NodeInterface;
+use Webmozart\Assert\Assert;
 
 final class DailyEmails implements \Countable {
 
@@ -14,6 +15,7 @@ final class DailyEmails implements \Countable {
   private function __construct(
     private array $emails,
   ) {
+    Assert::allIsInstanceOf($emails, NodeInterface::class);
   }
 
   public function count(): int {
diff --git a/modules/opd_podcast/src/Guests.php b/modules/opd_podcast/src/Guests.php
index 5ca818613..4f6bd29a3 100644
--- a/modules/opd_podcast/src/Guests.php
+++ b/modules/opd_podcast/src/Guests.php
@@ -4,6 +4,9 @@ declare(strict_types=1);
 
 namespace Drupal\opd_podcast;
 
+use Webmozart\Assert\Assert;
+
+
 /**
  * @implements \IteratorAggregate<Guest>
  */
@@ -13,6 +16,7 @@ final class Guests implements \Countable, \IteratorAggregate, \Stringable {
   * @param Guest[] $guests
   */
   private function __construct(private array $guests) {
+    Assert::allIsInstanceOf($guests, Guest::class);
   }
 
   public function count(): int {
diff --git a/modules/opd_presentations/src/Events.php b/modules/opd_presentations/src/Events.php
index b509d4de6..f79049686 100644
--- a/modules/opd_presentations/src/Events.php
+++ b/modules/opd_presentations/src/Events.php
@@ -4,6 +4,9 @@ declare(strict_types=1);
 
 namespace Drupal\opd_presentations;
 
+use Webmozart\Assert\Assert;
+
+
 /**
  * @implements \IteratorAggregate<Event>
  */
@@ -13,6 +16,7 @@ readonly final class Events implements \IteratorAggregate {
   * @param Event[] $events
   */
   private function __construct(private array $events) {
+    Assert::allIsInstanceOf($events, Event::class);
   }
 
   public function filter(\Closure $callback): self {