diff --git a/src/Symfony/Component/Cache/Traits/RedisTrait.php b/src/Symfony/Component/Cache/Traits/RedisTrait.php
index c27e0df689493..dabdde4e705ce 100644
--- a/src/Symfony/Component/Cache/Traits/RedisTrait.php
+++ b/src/Symfony/Component/Cache/Traits/RedisTrait.php
@@ -373,7 +373,7 @@ protected function doClear($namespace)
     {
         if ($this->redis instanceof \Predis\ClientInterface) {
             $prefix = $this->redis->getOptions()->prefix ? $this->redis->getOptions()->prefix->getPrefix() : '';
-            $prefixLen = \strlen($prefix);
+            $prefixLen = \strlen($prefix ?? '');
         }
 
         $cleared = true;