diff --git a/lightbox2.module b/lightbox2.module
index f775bee..cea0fc9 100644
--- a/lightbox2.module
+++ b/lightbox2.module
@@ -1649,6 +1649,10 @@ function lightbox2_views_api() {
 function lightbox2_filter_xss() {
   $allowed_tags = trim(variable_get('lightbox2_filter_xss_allowed_tags', 'p, br, a, em, strong, cite, code, ul, ol, li, dl, dt, dd, '));
   $allowed_tags = (empty($allowed_tags) ? array() : preg_split('/[,\s]+/', $allowed_tags));
+	if (!isset($_POST['string'])) {
+		drupal_json_output('');
+		exit;
+	}
   if (!empty($_POST['allowed_tags']) && $_POST['allowed_tags'] != 'undefined') {
     $allowed_tags = explode(',', $_POST['allowed_tags']);
     $output = filter_xss($_POST['string'], $allowed_tags);