diff --git a/htdocs/core/class/html.form.class.php b/htdocs/core/class/html.form.class.php
index 533a18524e3..77471d10124 100644
--- a/htdocs/core/class/html.form.class.php
+++ b/htdocs/core/class/html.form.class.php
@@ -2584,6 +2584,7 @@ class Form
 
 			$this->db->free($result);
 
+			include_once DOL_DOCUMENT_ROOT . '/core/lib/ajax.lib.php';
 			$out.=ajax_combobox($htmlname);
 
 			if (empty($outputmode)) return $out;