diff --git a/plugins/kolab_addressbook/drivers/carddav/carddav_contacts.php b/plugins/kolab_addressbook/drivers/carddav/carddav_contacts.php
index da8dc270..96a2b74f 100644
--- a/plugins/kolab_addressbook/drivers/carddav/carddav_contacts.php
+++ b/plugins/kolab_addressbook/drivers/carddav/carddav_contacts.php
@@ -235,7 +235,7 @@ class carddav_contacts extends rcube_addressbook
      *
      * @param mixed Search params to use in listing method, obtained by get_search_set()
      */
-    public function set_search_set($filter)
+    public function set_search_set($filter): void
     {
         $this->filter = $filter;
     }
@@ -253,7 +253,7 @@ class carddav_contacts extends rcube_addressbook
     /**
      * Reset saved results and search parameters
      */
-    public function reset()
+    public function reset(): void
     {
         $this->result = null;
         $this->filter = null;
diff --git a/plugins/kolab_addressbook/drivers/kolab/kolab_contacts.php b/plugins/kolab_addressbook/drivers/kolab/kolab_contacts.php
index 8c9216c1..9e4dce39 100644
--- a/plugins/kolab_addressbook/drivers/kolab/kolab_contacts.php
+++ b/plugins/kolab_addressbook/drivers/kolab/kolab_contacts.php
@@ -288,7 +288,7 @@ class kolab_contacts extends rcube_addressbook
      *
      * @param mixed Search params to use in listing method, obtained by get_search_set()
      */
-    public function set_search_set($filter)
+    public function set_search_set($filter): void
     {
         $this->filter = $filter;
     }
@@ -306,7 +306,7 @@ class kolab_contacts extends rcube_addressbook
     /**
      * Reset saved results and search parameters
      */
-    public function reset()
+    public function reset(): void
     {
         $this->result = null;
         $this->filter = null;