Fixed #16339

CHANGELOG.md

@@ -9,6 +9,7 @@
 - Fixed a bug where field conditions weren’t taking effect within Matrix fields set to inline-editable blocks mode, if the owner element didn’t support drafts. ([#16315](https://github.com/craftcms/cms/pull/16315))
 - Fixed a bug where Matrix fields’ entry types weren’t maintaining their original block type order when upgrading to Craft 5. ([#16314](https://github.com/craftcms/cms/issues/16314))
 - Fixed a bug where element card labels were getting cut off when wrapped. ([#16325](https://github.com/craftcms/cms/issues/16325))
+- Fixed a PHP error that could occur when eager-loading `owner` or `primaryOwner` on nested elements. ([#16339](https://github.com/craftcms/cms/issues/16339))
 
 ## 5.5.6.1 - 2024-12-11
 

src/base/NestedElementTrait.php

@@ -38,8 +38,13 @@ public static function eagerLoadingMap(array $sourceElements, string $handle): a
             case 'owner':
             case 'primaryOwner':
                 /** @var NestedElementInterface[] $sourceElements */
+                $ownerId = $sourceElements[0]->getOwnerId();
+                if (!$ownerId) {
+                    return false;
+                }
+
                 return [
-                    'elementType' => get_class(reset($sourceElements)),
+                    'elementType' => Craft::$app->getElements()->getElementTypeById($ownerId),
                     'map' => array_map(fn(NestedElementInterface $element) => [
                         'source' => $element->id,
                         'target' => match ($handle) {