diff --git a/lib/SimpleSAML/Metadata/MetaDataStorageHandlerPdo.php b/lib/SimpleSAML/Metadata/MetaDataStorageHandlerPdo.php
index 7731f3309a34fb5697e2af7fffb9d0b32a693560..a7b6fe2bb3c67838a071495ced63b15d8d30f272 100644
--- a/lib/SimpleSAML/Metadata/MetaDataStorageHandlerPdo.php
+++ b/lib/SimpleSAML/Metadata/MetaDataStorageHandlerPdo.php
@@ -153,8 +153,8 @@ class SimpleSAML_Metadata_MetaDataStorageHandlerPdo extends SimpleSAML_Metadata_
      */
     public function getMetaData($entityId, $set)      
     {       
-        assert('is_string($entityId)');
-        assert('is_string($set)');
+        assert(is_string($entityId));
+        assert(is_string($set));
 
         $tableName = $this->getTableName($set);