diff --git a/umap/static/umap/js/umap.core.js b/umap/static/umap/js/umap.core.js
index 30aaa4cb..c7afdb36 100644
--- a/umap/static/umap/js/umap.core.js
+++ b/umap/static/umap/js/umap.core.js
@@ -46,6 +46,7 @@ L.Util.escapeHTML = function (s) {
     return s.replace(/</gm, '&lt;');
 };
 L.Util.toHTML = function (r) {
+    if (!r) return '';
     var ii;
 
     // detect newline format