diff --git a/model/SqlResultProvider.php b/model/SqlResultProvider.php
index 9ec9048..631ea86 100644
--- a/model/SqlResultProvider.php
+++ b/model/SqlResultProvider.php
@@ -45,7 +45,7 @@ class SqlResultProvider implements iSqlResultItems
         $this->defineResultItems();
         $result_items_assoc = array();
         foreach ($this->result_items_base as $item) {
-            if (!isset($item->{$field})) {
+            if (!property_exists($item, $field)) {
                 throw new ErrorException('Undefined field. ' . $field);
             }
             if ($assoc_as_array) {