diff --git a/framework/core/src/Api/Schema/Relationship/ToMany.php b/framework/core/src/Api/Schema/Relationship/ToMany.php
index 24b7778ff..418338e3b 100644
--- a/framework/core/src/Api/Schema/Relationship/ToMany.php
+++ b/framework/core/src/Api/Schema/Relationship/ToMany.php
@@ -10,9 +10,22 @@
 namespace Flarum\Api\Schema\Relationship;
 
 use Flarum\Api\Schema\Concerns\FlarumRelationship;
+use InvalidArgumentException;
+use Tobyz\JsonApiServer\Context;
 use Tobyz\JsonApiServer\Laravel\Field\ToMany as BaseToMany;
 
 class ToMany extends BaseToMany
 {
     use FlarumRelationship;
+
+    public function serializeValue($value, Context $context): mixed
+    {
+        if ($value && ! is_array($value) && method_exists($value, 'toArray')) {
+            $value = $value->toArray();
+        } elseif ($value && ! is_array($value)) {
+            throw new InvalidArgumentException(sprintf('Relationship value [%s] must be an array', $this->name));
+        }
+
+        return parent::serializeValue($value, $context);
+    }
 }