diff --git a/src/Symfony/Component/HttpFoundation/Request.php b/src/Symfony/Component/HttpFoundation/Request.php
index 697bc6c3c3..d659d188ea 100644
--- a/src/Symfony/Component/HttpFoundation/Request.php
+++ b/src/Symfony/Component/HttpFoundation/Request.php
@@ -1382,10 +1382,10 @@ class Request
     public function getRequestFormat($default = 'html')
     {
         if (null === $this->format) {
-            $this->format = $this->get('_format', $default);
+            $this->format = $this->get('_format');
         }
 
-        return $this->format;
+        return null === $this->format ? $default : $this->format;
     }
 
     /**
diff --git a/src/Symfony/Component/HttpFoundation/Tests/RequestTest.php b/src/Symfony/Component/HttpFoundation/Tests/RequestTest.php
index 9000eb622b..2b852f5956 100644
--- a/src/Symfony/Component/HttpFoundation/Tests/RequestTest.php
+++ b/src/Symfony/Component/HttpFoundation/Tests/RequestTest.php
@@ -1401,6 +1401,11 @@ class RequestTest extends TestCase
         $request = new Request();
         $this->assertEquals('html', $request->getRequestFormat());
 
+        // Ensure that setting different default values over time is possible,
+        // aka. setRequestFormat determines the state.
+        $this->assertEquals('json', $request->getRequestFormat('json'));
+        $this->assertEquals('html', $request->getRequestFormat('html'));
+
         $request = new Request();
         $this->assertNull($request->getRequestFormat(null));