diff --git a/src/Symfony/Component/Cache/Adapter/AbstractAdapter.php b/src/Symfony/Component/Cache/Adapter/AbstractAdapter.php
index ca44fccad5d98..de534e343b213 100644
--- a/src/Symfony/Component/Cache/Adapter/AbstractAdapter.php
+++ b/src/Symfony/Component/Cache/Adapter/AbstractAdapter.php
@@ -15,7 +15,6 @@
 use Psr\Log\LoggerAwareInterface;
 use Psr\Log\LoggerAwareTrait;
 use Symfony\Component\Cache\CacheItem;
-use Symfony\Component\Cache\Exception\InvalidArgumentException;
 
 /**
  * @author Nicolas Grekas <p@tchwork.com>
@@ -31,7 +30,7 @@ abstract class AbstractAdapter implements AdapterInterface, LoggerAwareInterface
 
     protected function __construct($namespace = '', $defaultLifetime = 0)
     {
-        $this->namespace = $this->getId($namespace, true);
+        $this->namespace = '' === $namespace ? '' : $this->getId($namespace);
         $this->createCacheItem = \Closure::bind(
             function ($key, $value, $isHit) use ($defaultLifetime) {
                 $item = new CacheItem();
@@ -336,19 +335,9 @@ public function __destruct()
         }
     }
 
-    private function getId($key, $ns = false)
+    private function getId($key)
     {
-        if (!is_string($key)) {
-            throw new InvalidArgumentException(sprintf('Cache key must be string, "%s" given', is_object($key) ? get_class($key) : gettype($key)));
-        }
-        if (!isset($key[0]) && !$ns) {
-            throw new InvalidArgumentException('Cache key length must be greater than zero');
-        }
-        if (isset($key[strcspn($key, '{}()/\@:')])) {
-            throw new InvalidArgumentException('Cache key contains reserved characters {}()/\@:');
-        }
-
-        return $this->namespace.$key;
+        return $this->namespace.CacheItem::validateKey($key);
     }
 
     private function generateItems($items, &$keys)
diff --git a/src/Symfony/Component/Cache/Adapter/ArrayAdapter.php b/src/Symfony/Component/Cache/Adapter/ArrayAdapter.php
index d00488911c838..b6ca68b3ecfa5 100644
--- a/src/Symfony/Component/Cache/Adapter/ArrayAdapter.php
+++ b/src/Symfony/Component/Cache/Adapter/ArrayAdapter.php
@@ -15,7 +15,6 @@
 use Psr\Log\LoggerAwareInterface;
 use Psr\Log\LoggerAwareTrait;
 use Symfony\Component\Cache\CacheItem;
-use Symfony\Component\Cache\Exception\InvalidArgumentException;
 
 /**
  * @author Nicolas Grekas <p@tchwork.com>
@@ -74,7 +73,7 @@ public function getItem($key)
     public function getItems(array $keys = array())
     {
         foreach ($keys as $key) {
-            $this->validateKey($key);
+            CacheItem::validateKey($key);
         }
 
         return $this->generateItems($keys, time());
@@ -85,7 +84,7 @@ public function getItems(array $keys = array())
      */
     public function hasItem($key)
     {
-        return isset($this->expiries[$this->validateKey($key)]) && ($this->expiries[$key] >= time() || !$this->deleteItem($key));
+        return isset($this->expiries[CacheItem::validateKey($key)]) && ($this->expiries[$key] >= time() || !$this->deleteItem($key));
     }
 
     /**
@@ -103,7 +102,7 @@ public function clear()
      */
     public function deleteItem($key)
     {
-        unset($this->values[$this->validateKey($key)], $this->expiries[$key]);
+        unset($this->values[CacheItem::validateKey($key)], $this->expiries[$key]);
 
         return true;
     }
@@ -169,21 +168,6 @@ public function commit()
         return true;
     }
 
-    private function validateKey($key)
-    {
-        if (!is_string($key)) {
-            throw new InvalidArgumentException(sprintf('Cache key must be string, "%s" given', is_object($key) ? get_class($key) : gettype($key)));
-        }
-        if (!isset($key[0])) {
-            throw new InvalidArgumentException('Cache key length must be greater than zero');
-        }
-        if (isset($key[strcspn($key, '{}()/\@:')])) {
-            throw new InvalidArgumentException('Cache key contains reserved characters {}()/\@:');
-        }
-
-        return $key;
-    }
-
     private function generateItems(array $keys, $now)
     {
         $f = $this->createCacheItem;
diff --git a/src/Symfony/Component/Cache/Adapter/ProxyAdapter.php b/src/Symfony/Component/Cache/Adapter/ProxyAdapter.php
index e8befafac91f5..332638f76cfdd 100644
--- a/src/Symfony/Component/Cache/Adapter/ProxyAdapter.php
+++ b/src/Symfony/Component/Cache/Adapter/ProxyAdapter.php
@@ -14,7 +14,6 @@
 use Psr\Cache\CacheItemInterface;
 use Psr\Cache\CacheItemPoolInterface;
 use Symfony\Component\Cache\CacheItem;
-use Symfony\Component\Cache\Exception\InvalidArgumentException;
 
 /**
  * @author Nicolas Grekas <p@tchwork.com>
@@ -31,7 +30,7 @@ class ProxyAdapter implements AdapterInterface
     public function __construct(CacheItemPoolInterface $pool, $namespace = '', $defaultLifetime = 0)
     {
         $this->pool = $pool;
-        $this->namespace = $this->getId($namespace, true);
+        $this->namespace = '' === $namespace ? '' : $this->getId($namespace);
         $this->namespaceLen = strlen($namespace);
         $this->createCacheItem = \Closure::bind(
             function ($key, $value, $isHit) use ($defaultLifetime) {
@@ -192,18 +191,8 @@ public function getMisses()
         return $this->misses;
     }
 
-    private function getId($key, $ns = false)
+    private function getId($key)
     {
-        if (!is_string($key)) {
-            throw new InvalidArgumentException(sprintf('Cache key must be string, "%s" given', is_object($key) ? get_class($key) : gettype($key)));
-        }
-        if (!isset($key[0]) && !$ns) {
-            throw new InvalidArgumentException('Cache key length must be greater than zero');
-        }
-        if (isset($key[strcspn($key, '{}()/\@:')])) {
-            throw new InvalidArgumentException('Cache key contains reserved characters {}()/\@:');
-        }
-
-        return $this->namespace.$key;
+        return $this->namespace.CacheItem::validateKey($key);
     }
 }
diff --git a/src/Symfony/Component/Cache/CacheItem.php b/src/Symfony/Component/Cache/CacheItem.php
index 4ec571ac83d38..ce50b8e2ced0e 100644
--- a/src/Symfony/Component/Cache/CacheItem.php
+++ b/src/Symfony/Component/Cache/CacheItem.php
@@ -99,6 +99,30 @@ public function expiresAfter($time)
         return $this;
     }
 
+    /**
+     * Validates a cache key according to PSR-6.
+     *
+     * @param string $key The key to validate.
+     *
+     * @return string $key if it is valid.
+     *
+     * @throws InvalidArgumentException When $key is not valid.
+     */
+    public static function validateKey($key)
+    {
+        if (!is_string($key)) {
+            throw new InvalidArgumentException(sprintf('Cache key must be string, "%s" given', is_object($key) ? get_class($key) : gettype($key)));
+        }
+        if (!isset($key[0])) {
+            throw new InvalidArgumentException('Cache key length must be greater than zero');
+        }
+        if (isset($key[strcspn($key, '{}()/\@:')])) {
+            throw new InvalidArgumentException('Cache key contains reserved characters {}()/\@:');
+        }
+
+        return $key;
+    }
+
     /**
      * Internal logging helper.
      *