diff --git a/Adapter/Gaufrette/AbstractCmfMediaDoctrine.php b/Adapter/Gaufrette/AbstractCmfMediaDoctrine.php
index 4ef76e6..4341481 100644
--- a/Adapter/Gaufrette/AbstractCmfMediaDoctrine.php
+++ b/Adapter/Gaufrette/AbstractCmfMediaDoctrine.php
@@ -13,6 +13,7 @@
 use Symfony\Cmf\Bundle\MediaBundle\HierarchyInterface;
 use Symfony\Cmf\Bundle\MediaBundle\MediaInterface;
 use Symfony\Cmf\Bundle\MediaBundle\MediaManagerInterface;
+use Symfony\Cmf\Bundle\MediaBundle\MetadataInterface;
 
 /**
  * Cmf doctrine media adapter
@@ -133,6 +134,9 @@ public function write($key, $content)
     {
         if ($this->exists($key)) {
             $file = $this->find($key);
+            if (! $file instanceof FileInterface) {
+                return false;
+            }
         } else {
             $filePath = $this->computePath($key);
 
@@ -289,8 +293,8 @@ public function listKeys($prefix = '')
     /**
      * {@inheritDoc}
      *
-     * @throws RuntimeException If file cannot be found or cannot be written
-     *                          to
+     * @throws \RuntimeException If file cannot be found or cannot be written
+     *                           to
      */
     public function setMetadata($key, $metadata)
     {
@@ -300,6 +304,14 @@ public function setMetadata($key, $metadata)
             throw new \RuntimeException(sprintf('The file "%s" does not exist.', $key));
         }
 
+        if (! $file instanceof MetadataInterface) {
+            $type = is_object($file) ? get_class($file) : gettype($file);
+            throw new \InvalidArgumentException(sprintf(
+                'The class "%s" does not implement Symfony\Cmf\Bundle\MediaBundle\MetadataInterface',
+                $type
+            ));
+        }
+
         $file->setMetadata($metadata);
     }
 
@@ -351,6 +363,7 @@ public function setAutoFlush($bool)
      *
      * @param string|int $key Identifier.
      * @param boolean $dir directly try to find a directory
+     *
      * @return FileInterface
      */
     protected function find($key, $dir = false)
diff --git a/Form/DataTransformer/ModelToFileTransformer.php b/Form/DataTransformer/ModelToFileTransformer.php
index 85d28c8..6003a49 100644
--- a/Form/DataTransformer/ModelToFileTransformer.php
+++ b/Form/DataTransformer/ModelToFileTransformer.php
@@ -14,6 +14,13 @@ class ModelToFileTransformer implements DataTransformerInterface
 
     public function __construct($class)
     {
+        if (!is_subclass_of($class, 'Symfony\Cmf\Bundle\MediaBundle\FileInterface')) {
+            throw new \InvalidArgumentException(sprintf(
+                'The class "%s" does not implement Symfony\Cmf\Bundle\MediaBundle\FileInterface',
+                $class
+            ));
+        }
+
         $this->dataClass = $class;
     }
 
diff --git a/MetadataInterface.php b/MetadataInterface.php
index 7cdc159..9469382 100644
--- a/MetadataInterface.php
+++ b/MetadataInterface.php
@@ -20,7 +20,6 @@ public function getDescription();
 
     /**
      * @param string $description
-     * @return void
      */
     public function setDescription($description);
 
@@ -33,7 +32,6 @@ public function getCopyright();
 
     /**
      * @param string $copyright
-     * @return void
      */
     public function setCopyright($copyright);
 
@@ -46,7 +44,6 @@ public function getAuthorName();
 
     /**
      * @param string $author
-     * @return void
      */
     public function setAuthorName($author);
 
@@ -66,7 +63,9 @@ public function setMetadata(array $metadata);
 
     /**
      * @param string $name
-     * @param null   $default to be used if $name is not set in the metadata
+     * @param string $default to be used if $name is not set in the metadata
+     *
+     * @return string
      */
     public function getMetadataValue($name, $default = null);
 
@@ -74,7 +73,7 @@ public function getMetadataValue($name, $default = null);
      * The metadata value
      *
      * @param string $name
-     * @param mixed  $value
+     * @param string  $value
      */
     public function setMetadataValue($name, $value);
 
diff --git a/Model/Media.php b/Model/Media.php
index 28dfe2e..8db6cb4 100644
--- a/Model/Media.php
+++ b/Model/Media.php
@@ -27,7 +27,7 @@ class Media extends BaseMedia implements MetadataInterface
     protected $metadata;
 
     /**
-     * @param string $description
+     * {@inheritDoc}
      */
     public function setDescription($description)
     {
@@ -35,7 +35,7 @@ public function setDescription($description)
     }
 
     /**
-     * @return string
+     * {@inheritDoc}
      */
     public function getDescription()
     {
@@ -43,7 +43,7 @@ public function getDescription()
     }
 
     /**
-     * @param string $copyright
+     * {@inheritDoc}
      */
     public function setCopyright($copyright)
     {
@@ -51,7 +51,7 @@ public function setCopyright($copyright)
     }
 
     /**
-     * @return string
+     * {@inheritDoc}
      */
     public function getCopyright()
     {
@@ -59,7 +59,7 @@ public function getCopyright()
     }
 
     /**
-     * @param string $authorName
+     * {@inheritDoc}
      */
     public function setAuthorName($authorName)
     {
@@ -67,7 +67,7 @@ public function setAuthorName($authorName)
     }
 
     /**
-     * @return string
+     * {@inheritDoc}
      */
     public function getAuthorName()
     {
@@ -75,7 +75,7 @@ public function getAuthorName()
     }
 
     /**
-     * @param array $metadata
+     * {@inheritDoc}
      */
     public function setMetadata(array $metadata)
     {
@@ -83,7 +83,7 @@ public function setMetadata(array $metadata)
     }
 
     /**
-     * @return array
+     * {@inheritDoc}
      */
     public function getMetadata()
     {
@@ -91,18 +91,15 @@ public function getMetadata()
     }
 
     /**
-     * @param string $name
-     * @param null $default
-     * @return null
+     * {@inheritDoc}
      */
     public function getMetadataValue($name, $default = null)
     {
-        return isset($metadata[$name]) ? $metadata[$name] : $default;
+        return isset($this->metadata[$name]) ? $this->metadata[$name] : $default;
     }
 
     /**
-     * @param string $name
-     * @param mixed  $value
+     * {@inheritDoc}
      */
     public function setMetadataValue($name, $value)
     {
@@ -114,8 +111,6 @@ public function setMetadataValue($name, $value)
      */
     public function unsetMetadataValue($name)
     {
-        $metadata = $this->getMetadata();
-        unset($metadata[$name]);
-        $this->setMetadata($metadata);
+        unset($this->metadata[$name]);
     }
 }