diff --git a/plugins/libkolab/lib/kolab_utils.php b/plugins/libkolab/lib/kolab_utils.php
index 2f4d0dfa..fd6badba 100644
--- a/plugins/libkolab/lib/kolab_utils.php
+++ b/plugins/libkolab/lib/kolab_utils.php
@@ -1,120 +1,122 @@
 <?php
 
 /**
  * Utility class providing unified functionality for other plugins.
  *
  * @version @package_version@
  * @author Thomas Bruederli <bruederli@kolabsys.com>
  * @author Aleksander Machniak <machniak@kolabsys.com>
  *
  * Copyright (C) 2012-2018, Kolab Systems AG <contact@kolabsys.com>
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU Affero General Public License as
  * published by the Free Software Foundation, either version 3 of the
  * License, or (at your option) any later version.
  *
  * This program is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  * GNU Affero General Public License for more details.
  *
  * You should have received a copy of the GNU Affero General Public License
  * along with this program. If not, see <http://www.gnu.org/licenses/>.
  */
 
 class kolab_utils
 {
     public static function folder_form($form, $folder, $domain, $hidden_fields = [], $no_acl = false)
     {
         $rcmail = rcube::get_instance();
 
         // add folder ACL tab
-        if (!$no_acl && is_string($folder) && strlen($folder)) {
-            $form['sharing'] = [
-                'name'    => rcube::Q($rcmail->gettext('libkolab.tabsharing')),
-                'content' => self::folder_acl_form($folder),
-            ];
+        if (!$no_acl && $folder) {
+            if (($folder instanceof kolab_storage_dav_folder) || (is_string($folder) && strlen($folder))) {
+                $form['sharing'] = [
+                    'name'    => rcube::Q($rcmail->gettext('libkolab.tabsharing')),
+                    'content' => self::folder_acl_form($folder),
+                ];
+            }
         }
 
         $form_html = '';
 
         if (is_array($hidden_fields)) {
             foreach ($hidden_fields as $field) {
                 $hiddenfield = new html_hiddenfield($field);
                 $form_html .= $hiddenfield->show() . "\n";
             }
         }
 
         // create form output
         foreach ($form as $tab) {
             if (isset($tab['fields']) && is_array($tab['fields']) && empty($tab['content'])) {
                 $table = new html_table(['cols' => 2, 'class' => 'propform']);
                 foreach ($tab['fields'] as $col => $colprop) {
                     $label = !empty($colprop['label']) ? $colprop['label'] : $rcmail->gettext("$domain.$col");
 
                     $table->add('title', html::label($colprop['id'], rcube::Q($label)));
                     $table->add(null, $colprop['value']);
                 }
                 $content = $table->show();
             } else {
                 $content = $tab['content'];
             }
 
             if (!empty($content)) {
                 $form_html .= html::tag('fieldset', null, html::tag('legend', null, rcube::Q($tab['name'])) . $content) . "\n";
             }
         }
 
         return $form_html;
     }
 
     /**
      * Handler for ACL form template object
      *
      * @param string|kolab_storage_dav_folder $folder DAV folder object or IMAP folder name
      *
      * @return string HTML content
      */
     private static function folder_acl_form($folder)
     {
         if ($folder instanceof kolab_storage_dav_folder) {
             return self::folder_dav_acl_form($folder);
         }
 
         $rcmail  = rcube::get_instance();
         $storage = $rcmail->get_storage();
         $options = $storage->folder_info($folder);
 
         $rcmail->plugins->load_plugin('acl', true);
 
         // get sharing UI from acl plugin
         $acl = $rcmail->plugins->exec_hook('folder_form', [
                 'form'    => [],
                 'options' => $options,
                 'name'    => $folder,
         ]);
 
         if (!empty($acl['form']['sharing']['content'])) {
             return $acl['form']['sharing']['content'];
         }
 
         return html::div('hint', $rcmail->gettext('libkolab.aclnorights'));
     }
 
     /**
      * Handler for DAV ACL form template object
      *
      * @param kolab_storage_dav_folder $folder DAV folder object
      *
      * @return string HTML content
      */
     private static function folder_dav_acl_form($folder)
     {
         if ($form = kolab_dav_acl::form($folder)) {
             return $form;
         }
 
         return html::div('hint', rcmail::get_instance()->gettext('libkolab.aclnorights'));
     }
 }