diff --git a/src/Symfony/Component/HttpFoundation/Request.php b/src/Symfony/Component/HttpFoundation/Request.php
index ab22200fc941b..39f6cfee4e3bf 100644
--- a/src/Symfony/Component/HttpFoundation/Request.php
+++ b/src/Symfony/Component/HttpFoundation/Request.php
@@ -1333,7 +1333,7 @@ public static function getMimeTypes(string $format)
     public function getFormat(?string $mimeType)
     {
         $canonicalMimeType = null;
-        if (false !== $pos = strpos($mimeType, ';')) {
+        if ($mimeType && false !== $pos = strpos($mimeType, ';')) {
             $canonicalMimeType = trim(substr($mimeType, 0, $pos));
         }