diff --git a/model/Concept.php b/model/Concept.php
index 3f31ee282..1e4b8dec6 100644
--- a/model/Concept.php
+++ b/model/Concept.php
@@ -286,7 +286,7 @@ public function getProperties()
     }
 
     foreach ($long_uris as &$prop) {
-      if (EasyRdf_Namespace::shorten($prop)) // shortening property labels if possible
+      if (EasyRdf_Namespace::shorten($prop) !== null) // shortening property labels if possible
         $prop = $sprop = EasyRdf_Namespace::shorten($prop);
       else
         $sprop = "<$prop>"; // EasyRdf requires full URIs to be in angle brackets