diff --git a/File/MimeType/FileBinaryMimeTypeGuesser.php b/File/MimeType/FileBinaryMimeTypeGuesser.php
index 34e015ee5..b75242afd 100644
--- a/File/MimeType/FileBinaryMimeTypeGuesser.php
+++ b/File/MimeType/FileBinaryMimeTypeGuesser.php
@@ -74,7 +74,7 @@ public function guess($path)
         }
 
         if (!self::isSupported()) {
-            return;
+            return null;
         }
 
         ob_start();
@@ -84,14 +84,14 @@ public function guess($path)
         if ($return > 0) {
             ob_end_clean();
 
-            return;
+            return null;
         }
 
         $type = trim(ob_get_clean());
 
         if (!preg_match('#^([a-z0-9\-]+/[a-z0-9\-\.]+)#i', $type, $match)) {
             // it's not a type, but an error message
-            return;
+            return null;
         }
 
         return $match[1];
diff --git a/File/MimeType/FileinfoMimeTypeGuesser.php b/File/MimeType/FileinfoMimeTypeGuesser.php
index 62feda2ee..fc4bc4502 100644
--- a/File/MimeType/FileinfoMimeTypeGuesser.php
+++ b/File/MimeType/FileinfoMimeTypeGuesser.php
@@ -57,11 +57,11 @@ public function guess($path)
         }
 
         if (!self::isSupported()) {
-            return;
+            return null;
         }
 
         if (!$finfo = new \finfo(FILEINFO_MIME_TYPE, $this->magicFile)) {
-            return;
+            return null;
         }
 
         return $finfo->file($path);
diff --git a/File/MimeType/MimeTypeGuesserInterface.php b/File/MimeType/MimeTypeGuesserInterface.php
index 5ac1acb82..e46e78eef 100644
--- a/File/MimeType/MimeTypeGuesserInterface.php
+++ b/File/MimeType/MimeTypeGuesserInterface.php
@@ -26,7 +26,7 @@ interface MimeTypeGuesserInterface
      *
      * @param string $path The path to the file
      *
-     * @return string The mime type or NULL, if none could be guessed
+     * @return string|null The mime type or NULL, if none could be guessed
      *
      * @throws FileNotFoundException If the file does not exist
      * @throws AccessDeniedException If the file could not be read
diff --git a/RequestStack.php b/RequestStack.php
index 885d78a50..244a77d63 100644
--- a/RequestStack.php
+++ b/RequestStack.php
@@ -47,7 +47,7 @@ public function push(Request $request)
     public function pop()
     {
         if (!$this->requests) {
-            return;
+            return null;
         }
 
         return array_pop($this->requests);
@@ -73,7 +73,7 @@ public function getCurrentRequest()
     public function getMasterRequest()
     {
         if (!$this->requests) {
-            return;
+            return null;
         }
 
         return $this->requests[0];
@@ -95,7 +95,7 @@ public function getParentRequest()
         $pos = \count($this->requests) - 2;
 
         if (!isset($this->requests[$pos])) {
-            return;
+            return null;
         }
 
         return $this->requests[$pos];