diff --git a/htdocs/core/lib/functions.lib.php b/htdocs/core/lib/functions.lib.php
index 0ac5ea25bb0..31297c84a9a 100644
--- a/htdocs/core/lib/functions.lib.php
+++ b/htdocs/core/lib/functions.lib.php
@@ -13497,7 +13497,7 @@ function jsonOrUnserialize($stringtodecode)
 /**
  * forgeSQLFromUniversalSearchCriteria
  *
- * @param 	string		$filter		String with universal search string. Must be '(aaa:bbb:ccc) OR (ddd:eeee:fff) ...' with
+ * @param 	?string		$filter		String with universal search string. Must be '(aaa:bbb:ccc) OR (ddd:eeee:fff) ...' with
  * 									aaa is a field name (with alias or not) and
  * 									bbb is one of this operator '=', '<', '>', '<=', '>=', '!=', 'in', 'notin', 'like', 'notlike', 'is', 'isnot'.
  * 									ccc must not contains ( or )
@@ -13514,7 +13514,7 @@ function forgeSQLFromUniversalSearchCriteria($filter, &$errorstr = '', $noand =
 {
 	global $db, $user;
 
-	if ($filter === '') {
+	if (is_null($filter) || $filter === '') {
 		return '';
 	}
 	if (!preg_match('/^\(.*\)$/', $filter)) {    // If $filter does not start and end with ()