diff --git a/plugins/calendar/calendar_ui.js b/plugins/calendar/calendar_ui.js
index 7630ccd7..6232b7e4 100644
--- a/plugins/calendar/calendar_ui.js
+++ b/plugins/calendar/calendar_ui.js
@@ -38,7 +38,7 @@ function rcube_calendar_ui(settings)
     this.selected_event = null;
     this.selected_calendar = null;
     this.search_request = null;
-    this.saving_lock;
+    this.saving_lock = null;
     this.calendars = {};
     this.quickview_sources = [];
 
diff --git a/plugins/kolab_files/kolab_files.js b/plugins/kolab_files/kolab_files.js
index 39b931a8..620a7ed2 100644
--- a/plugins/kolab_files/kolab_files.js
+++ b/plugins/kolab_files/kolab_files.js
@@ -2177,7 +2177,7 @@ function kolab_files_ui()
   {
     return $('<a>').attr({
         title: rcmail.gettext('kolab_files.listpermanent'),
-        class: 'subscription' + (subscribed ? ' subscribed' : ''),
+        'class': 'subscription' + (subscribed ? ' subscribed' : ''),
         'aria-checked': subscribed,
         role: 'checkbox'
     });
diff --git a/plugins/kolab_notes/notes.js b/plugins/kolab_notes/notes.js
index f4a57cc4..be9097d7 100644
--- a/plugins/kolab_notes/notes.js
+++ b/plugins/kolab_notes/notes.js
@@ -122,7 +122,7 @@ function rcube_kolab_notes_ui(settings)
                     rcmail.enable_command('createnote', has_permission(me.notebooks[id], 'i'));
                     rcmail.enable_command('list-edit', has_permission(me.notebooks[id], 'a'));
                     rcmail.enable_command('list-delete', has_permission(me.notebooks[id], 'xa'));
-                    rcmail.enable_command('list-remove', !me.notebooks[id].default);
+                    rcmail.enable_command('list-remove', !me.notebooks[id]['default']);
                     fetch_notes(id);  // sets me.selected_list
                 },
                 function(){
@@ -142,7 +142,7 @@ function rcube_kolab_notes_ui(settings)
             }
         });
         notebookslist.addEventListener('remove', function(p) {
-            if (me.notebooks[p.id] && !me.notebooks[p.id].default) {
+            if (me.notebooks[p.id] && !me.notebooks[p.id]['default']) {
                 list_remove(p.id);
             }
         });
diff --git a/plugins/tasklist/tasklist.js b/plugins/tasklist/tasklist.js
index 3c8f2a32..b39d9686 100644
--- a/plugins/tasklist/tasklist.js
+++ b/plugins/tasklist/tasklist.js
@@ -106,8 +106,8 @@ function rcube_tasklist_ui(settings)
 
     /*  public members  */
     this.tasklists = rcmail.env.tasklists;
-    this.selected_task;
-    this.selected_list;
+    this.selected_task = null;
+    this.selected_list = null;
 
     /*  public methods  */
     this.init = init;
@@ -150,7 +150,7 @@ function rcube_tasklist_ui(settings)
                 me.selected_list = settings.selected_list;
                 $(rcmail.gui_objects.tasklistslist).find("input[value='"+settings.selected_list+"']").prop('checked', true);
             }
-            if (me.tasklists[id].editable && (!me.selected_list || me.tasklists[id].default || (me.tasklists[id].active && !me.tasklists[me.selected_list].active))) {
+            if (me.tasklists[id].editable && (!me.selected_list || me.tasklists[id]['default'] || (me.tasklists[id].active && !me.tasklists[me.selected_list].active))) {
                 me.selected_list = id;
             }
         }