diff --git a/libraries/classes/Display/Results.php b/libraries/classes/Display/Results.php
index dba6267613..1105148ee0 100644
--- a/libraries/classes/Display/Results.php
+++ b/libraries/classes/Display/Results.php
@@ -4701,7 +4701,7 @@ class Results
 
         $column_for_last_row = mb_strtoupper(
             mb_substr(
-                $column_for_last_row,
+                (string) $column_for_last_row,
                 0,
                 $GLOBALS['cfg']['LimitChars']
             ) . '...'