diff --git a/.eslintrc.json b/.eslintrc.json
index 6aef9e75..ca6715f2 100644
--- a/.eslintrc.json
+++ b/.eslintrc.json
@@ -5,7 +5,7 @@
     "es6": true
   },
   "parserOptions": {
-    "ecmaVersion": 2020,
+    "ecmaVersion": 2021,
     "sourceType": "module"
   }
 }
diff --git a/umap/static/umap/js/modules/slideshow.js b/umap/static/umap/js/modules/slideshow.js
index 496d3537..c00b60a7 100644
--- a/umap/static/umap/js/modules/slideshow.js
+++ b/umap/static/umap/js/modules/slideshow.js
@@ -18,6 +18,7 @@ export default class Slideshow extends WithTemplate {
     this._umap = umap
     this._id = null
     this.CLASSNAME = 'umap-slideshow-active'
+    this._umap.properties.slideshow ??= {}
     this.load()
     this._current = null