diff --git a/app/Http/Controllers/Oai/RequestController.php b/app/Http/Controllers/Oai/RequestController.php
index e5b77aa..0162bec 100644
--- a/app/Http/Controllers/Oai/RequestController.php
+++ b/app/Http/Controllers/Oai/RequestController.php
@@ -149,7 +149,7 @@ class RequestController extends Controller
         $repositoryName = "Tethys RDR";
         $repIdentifier = "tethys.geologie.ac.at";
         $sampleIdentifier = "oai:" . $repIdentifier . ":27";//$this->_configuration->getSampleIdentifier();
-        $earliestDateFromDb = optional(Dataset::earliestPublicationDate())->server_date_published;
+        $earliestDateFromDb = Dataset::earliestPublicationDate() != null ?  Dataset::earliestPublicationDate()->server_date_published: null;
 
         // set parameters for oai-pmh.xslt
         $this->_proc->setParameter('', 'email', $email);
diff --git a/app/Models/Dataset.php b/app/Models/Dataset.php
index 3d741c5..238b1b8 100644
--- a/app/Models/Dataset.php
+++ b/app/Models/Dataset.php
@@ -258,17 +258,18 @@ class Dataset extends Model
      *
      * @return \Carbon\Carbon\Date
      */
-    public function scopeEarliestPublicationDate($query, string $column = null)
+    public static function earliestPublicationDate(string $column = null)
     {
         if (!$column) {
             $column = self::PUBLISHED_AT;
         }
-        return $query->select('server_date_published')
+        $result = Dataset::select('server_date_published')
             ->where('server_date_published', '<>', null)
             ->where('server_state', 'published')
             ->orderBy('server_date_published', 'asc')
             ->first();
             //->server_date_published;
+        return $result;
     }
 
     public function setServerState($targetType)