diff --git a/Service/PresentationService.php b/Service/PresentationService.php
index e97db6f..71701e1 100644
--- a/Service/PresentationService.php
+++ b/Service/PresentationService.php
@@ -476,7 +476,7 @@ private function getMetadata(\Subugoe\IIIFModel\Model\Document $document): array
             if (!empty($value)) {
                 $data = new Metadata();
                 $data
-                    ->setLabel($key)
+                    ->setLabel((string) $key)
                     ->setValue($value);
                 $metadata[] = $data;
             }