diff --git a/core/modules/views/src/Plugin/views/style/StylePluginBase.php b/core/modules/views/src/Plugin/views/style/StylePluginBase.php
index ccfd1ba..467a90c 100644
--- a/core/modules/views/src/Plugin/views/style/StylePluginBase.php
+++ b/core/modules/views/src/Plugin/views/style/StylePluginBase.php
@@ -599,6 +599,7 @@ public function renderGrouping($records, $groupings = array(), $group_rendered =
           }
 
           // Create the group if it does not exist yet.
+          $grouping = (string) $grouping;
           if (empty($set[$grouping])) {
             $set[$grouping]['group'] = $group_content;
             $set[$grouping]['rows'] = array();