diff --git a/modules/monitoring/library/Monitoring/Backend/Ido/Query/IdoQuery.php b/modules/monitoring/library/Monitoring/Backend/Ido/Query/IdoQuery.php
index 3dd34dd70..0d80bf5ee 100644
--- a/modules/monitoring/library/Monitoring/Backend/Ido/Query/IdoQuery.php
+++ b/modules/monitoring/library/Monitoring/Backend/Ido/Query/IdoQuery.php
@@ -572,7 +572,7 @@ abstract class IdoQuery extends DbQuery
         $column = $subQuery->aliasToColumnName($alias);
         if (isset($this->caseInsensitiveColumns[$subQuery->aliasToTableName($alias)][$alias])) {
             $column = 'LOWER( ' . $column . ' )';
-            $subQueryFilter->setExpression(array_map('strtolower', $subQueryFilter->getExpression()));
+            $subQueryFilter->setExpression(array_map('strtolower', (array) $subQueryFilter->getExpression()));
         }
 
         $additional = null;