diff --git a/plugins/calendar/drivers/kolab/kolab_calendar.php b/plugins/calendar/drivers/kolab/kolab_calendar.php
index 816a9444..50b06ed0 100644
--- a/plugins/calendar/drivers/kolab/kolab_calendar.php
+++ b/plugins/calendar/drivers/kolab/kolab_calendar.php
@@ -67,9 +67,11 @@ class kolab_calendar
       }
       else {
         $acl = $this->storage->_folder->getACL();
-        $acl = $acl[$_SESSION['username']];
-        if (strpos($acl, 'i') !== false)
-          $this->readonly = false;
+        if (is_array($acl)) {
+          $acl = $acl[$_SESSION['username']];
+          if (strpos($acl, 'i') !== false)
+            $this->readonly = false;
+        }
       }
     }
   }
diff --git a/plugins/kolab_addressbook/lib/rcube_kolab_contacts.php b/plugins/kolab_addressbook/lib/rcube_kolab_contacts.php
index 5b897c27..6cc0289b 100644
--- a/plugins/kolab_addressbook/lib/rcube_kolab_contacts.php
+++ b/plugins/kolab_addressbook/lib/rcube_kolab_contacts.php
@@ -128,11 +128,13 @@ class rcube_kolab_contacts extends rcube_addressbook
             }
             else {
                 $acl = $this->storagefolder->getACL();
-                $acl = $acl[$_SESSION['username']];
-                if (strpos($acl, 'i') !== false)
-                    $this->readonly = false;
-                if (strpos($acl, 'a') !== false || strpos($acl, 'x') !== false)
-                    $this->editable = true;
+                if (is_array($acl)) {
+                    $acl = $acl[$_SESSION['username']];
+                    if (strpos($acl, 'i') !== false)
+                        $this->readonly = false;
+                    if (strpos($acl, 'a') !== false || strpos($acl, 'x') !== false)
+                        $this->editable = true;
+                }
             }
         }
     }