diff --git a/libraries/plugins/auth/AuthenticationCookie.class.php b/libraries/plugins/auth/AuthenticationCookie.class.php
index 04b8638ffc..20966fb28b 100644
--- a/libraries/plugins/auth/AuthenticationCookie.class.php
+++ b/libraries/plugins/auth/AuthenticationCookie.class.php
@@ -793,7 +793,7 @@ class AuthenticationCookie extends AuthenticationPlugin
                 true
             );
         }
-        if (strlen($this->_cookie_iv) < $this->getIVSize()) {
+        if (mb_strlen($this->_cookie_iv,'8bit') < $this->getIVSize()) {
                 $this->createIV();
         }