diff --git a/src/Symfony/Component/Cache/Adapter/AbstractAdapter.php b/src/Symfony/Component/Cache/Adapter/AbstractAdapter.php
index 883a94ba9bae..51c4b6664c74 100644
--- a/src/Symfony/Component/Cache/Adapter/AbstractAdapter.php
+++ b/src/Symfony/Component/Cache/Adapter/AbstractAdapter.php
@@ -29,12 +29,14 @@ abstract class AbstractAdapter implements AdapterInterface, CacheInterface, Logg
     use AbstractAdapterTrait;
     use ContractsTrait;
 
+    protected const NS_SEPARATOR = ':';
+
     private static $apcuSupported;
     private static $phpFilesSupported;
 
     protected function __construct(string $namespace = '', int $defaultLifetime = 0)
     {
-        $this->namespace = '' === $namespace ? '' : CacheItem::validateKey($namespace).':';
+        $this->namespace = '' === $namespace ? '' : CacheItem::validateKey($namespace).static::NS_SEPARATOR;
         if (null !== $this->maxIdLength && \strlen($namespace) > $this->maxIdLength - 24) {
             throw new InvalidArgumentException(sprintf('Namespace must be %d chars max, %d given ("%s")', $this->maxIdLength - 24, \strlen($namespace), $namespace));
         }
diff --git a/src/Symfony/Component/Cache/Adapter/Psr16Adapter.php b/src/Symfony/Component/Cache/Adapter/Psr16Adapter.php
index a14ee082be1e..ffe68b7c7a5b 100644
--- a/src/Symfony/Component/Cache/Adapter/Psr16Adapter.php
+++ b/src/Symfony/Component/Cache/Adapter/Psr16Adapter.php
@@ -25,6 +25,8 @@ class Psr16Adapter extends AbstractAdapter implements PruneableInterface, Resett
 {
     use ProxyTrait;
 
+    protected const NS_SEPARATOR = '_';
+
     private $miss;
 
     public function __construct(CacheInterface $pool, string $namespace = '', int $defaultLifetime = 0)
diff --git a/src/Symfony/Component/Cache/Tests/Adapter/Psr16AdapterTest.php b/src/Symfony/Component/Cache/Tests/Adapter/Psr16AdapterTest.php
index 19907ddf7873..e67a9962ec6d 100644
--- a/src/Symfony/Component/Cache/Tests/Adapter/Psr16AdapterTest.php
+++ b/src/Symfony/Component/Cache/Tests/Adapter/Psr16AdapterTest.php
@@ -14,6 +14,7 @@
 use Symfony\Component\Cache\Adapter\FilesystemAdapter;
 use Symfony\Component\Cache\Adapter\Psr16Adapter;
 use Symfony\Component\Cache\Psr16Cache;
+use Symfony\Component\Debug\BufferingLogger;
 
 /**
  * @group time-sensitive
@@ -28,4 +29,20 @@ public function createCachePool($defaultLifetime = 0)
     {
         return new Psr16Adapter(new Psr16Cache(new FilesystemAdapter()), '', $defaultLifetime);
     }
+
+    public function testValidCacheKeyWithNamespace()
+    {
+        $logger = new BufferingLogger();
+        $cache = new Psr16Adapter(new Psr16Cache(new FilesystemAdapter()), 'some_namespace', 0);
+        $cache->setLogger($logger);
+        $this->assertSame('foo', $cache->get('my_key', function () {
+            return 'foo';
+        }));
+        $logs = $logger->cleanLogs();
+        foreach ($logs as $log) {
+            if ('warning' === $log[0] || 'error' === $log[0]) {
+                $this->fail('An error was triggered while caching key with a namespace: '.$log[1]);
+            }
+        }
+    }
 }
diff --git a/src/Symfony/Component/Cache/composer.json b/src/Symfony/Component/Cache/composer.json
index 05bed1cf7548..d3ab17689cdb 100644
--- a/src/Symfony/Component/Cache/composer.json
+++ b/src/Symfony/Component/Cache/composer.json
@@ -35,6 +35,7 @@
         "predis/predis": "~1.1",
         "psr/simple-cache": "^1.0",
         "symfony/config": "~4.2",
+        "symfony/debug": "^4",
         "symfony/dependency-injection": "~3.4|~4.1",
         "symfony/var-dumper": "^4.1.1"
     },