diff --git a/htdocs/core/lib/functions.lib.php b/htdocs/core/lib/functions.lib.php
index 8413e5d893a..ac3ec79e418 100644
--- a/htdocs/core/lib/functions.lib.php
+++ b/htdocs/core/lib/functions.lib.php
@@ -7128,6 +7128,8 @@ function getTitleFieldOfList($name, $thead = 0, $file = "", $field = "", $begin
 		$prefix .= 'right '; // For backward compatibility
 	}
 
+	$tooltip = (string) $tooltip;	// In case $tooltip is null
+
 	$sortorder = strtoupper((string) $sortorder);
 	$out = '';
 	$sortimg = '';