diff --git a/tests/src/Assertions/EntityTrait.php b/tests/src/Assertions/EntityTrait.php
index d9380b7..a25e8eb 100644
--- a/tests/src/Assertions/EntityTrait.php
+++ b/tests/src/Assertions/EntityTrait.php
@@ -10,7 +10,7 @@ trait EntityTrait {
   /**
    * Asserts an entity with a specific bundle exists.
    */
-  public function assertEntityExists(string $entity_type, string $bundle = NULL) {
+  public function assertEntityExists(string $entity_type, ?string $bundle = NULL) {
     $bundles = \Drupal::service('entity_type.bundle.info')->getBundleInfo($entity_type);
 
     $bundle = $bundle ?? $entity_type;