diff --git a/classes/migration/upgrade/OJSv3_3_0UpgradeMigration.inc.php b/classes/migration/upgrade/OJSv3_3_0UpgradeMigration.inc.php
index 8b1feb6f43a..c9952cf085b 100644
--- a/classes/migration/upgrade/OJSv3_3_0UpgradeMigration.inc.php
+++ b/classes/migration/upgrade/OJSv3_3_0UpgradeMigration.inc.php
@@ -177,7 +177,7 @@ private function _toJSON($row, $tableName, $searchBy, $valueToConvert)
 		}
 
 		if (!count($primaryKeys)) {
-			foreach (array_keys($row) as $column) {
+			foreach (array_keys(get_object_vars($row)) as $column) {
 				if (substr($column, -3, '_id')) {
 					$primaryKeys[] = $column;
 				}