diff -r 17f10b56c079 -r 8fddc113095e server/src/app/Models/WebResource.php
--- a/server/src/app/Models/WebResource.php	Thu Jun 02 18:24:19 2016 +0200
+++ b/server/src/app/Models/WebResource.php	Sun Jun 05 00:28:45 2016 +0200
@@ -16,17 +16,17 @@
         parent::__construct($uri, $graph);
     }
 
-    private $format = -1;
+    private $format = false;
 
     abstract protected function doClearMemoizationCache();
 
     public function clearMemoizationCache() {
-        $this->format = -1;
+        $this->format = false;
         $this->doClearMemoizationCache();
     }
 
     public function getFormat() {
-        if($this->format === -1) {
+        if($this->format === false) {
             $format = $this->getLiteral("<http://purl.org/dc/elements/1.1/format>");
             $this->format = is_null($format)?null:$format->getValue();
         }