diff --git a/CHANGELOG.md b/CHANGELOG.md
index 91c8827a7e1..640f5bdfcd5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -27,6 +27,7 @@
 - Fix potential HTTP protocol version mismatch (#8982)
 - Fix PHP8 fatal error when parsing a malformed BODYSTRUCTURE (#9171)
 - Fix duplicated Inbox folder on IMAP servers that do not use Inbox folder with all capital letters (#9166)
+- Fix PHP warnings (#9174)
 
 ## Release 1.6.4
 
diff --git a/plugins/managesieve/lib/Roundcube/rcube_sieve_engine.php b/plugins/managesieve/lib/Roundcube/rcube_sieve_engine.php
index 0138343f4a7..7e77a2bbc74 100644
--- a/plugins/managesieve/lib/Roundcube/rcube_sieve_engine.php
+++ b/plugins/managesieve/lib/Roundcube/rcube_sieve_engine.php
@@ -727,7 +727,7 @@ function save()
                         $this->form['tests'][$i]['type'] = $sizeop;
                         $this->form['tests'][$i]['arg']  = $sizetarget;
 
-                        if ($sizetarget == '') {
+                        if ($sizetarget === '') {
                             $this->errors['tests'][$i]['sizetarget'] = $this->plugin->gettext('cannotbeempty');
                         }
                         else if (!preg_match('/^[0-9]+(K|M|G)?$/i', $sizetarget.$sizeitem, $m)) {
@@ -1067,8 +1067,8 @@ function save()
 
                 case 'redirect':
                 case 'redirect_copy':
-                    $target = $this->strip_value(isset($act_targets[$idx]) ? $act_targets[$idx] : null);
-                    $domain = $this->strip_value(isset($domain_targets[$idx]) ? $domain_targets[$idx] : null);
+                    $target = $this->strip_value($act_targets[$idx] ?? null);
+                    $domain = $this->strip_value($domain_targets[$idx] ?? null);
 
                     // force one of the configured domains
                     $domains = (array) $this->rc->config->get('managesieve_domains');
@@ -1082,7 +1082,7 @@ function save()
 
                     $this->form['actions'][$i]['target'] = $target;
 
-                    if ($target == '') {
+                    if ($target === '') {
                         $this->errors['actions'][$i]['target'] = $this->plugin->gettext('cannotbeempty');
                     }
                     else if (!rcube_utils::check_email($target)) {
@@ -2780,6 +2780,8 @@ protected function strip_value($str, $allow_html = false, $trim = true)
             return $str;
         }
 
+        $str = (string) $str;
+
         if (!$allow_html) {
             $str = strip_tags($str);
         }