diff --git a/libraries/classes/InsertEdit.php b/libraries/classes/InsertEdit.php
index d2f09c887f..d679eff375 100644
--- a/libraries/classes/InsertEdit.php
+++ b/libraries/classes/InsertEdit.php
@@ -2420,7 +2420,7 @@ class InsertEdit
             $foreigner['foreign_table']
         );
         // Field to display from the foreign table?
-        if ($display_field !== null && strlen($display_field) > 0) {
+        if (is_string($display_field) && strlen($display_field) > 0) {
             $dispsql = 'SELECT ' . Util::backquote($display_field)
                 . ' FROM ' . Util::backquote($foreigner['foreign_db'])
                 . '.' . Util::backquote($foreigner['foreign_table'])