diff --git a/Request.php b/Request.php
index a5e50eb79..33f4efb5e 100644
--- a/Request.php
+++ b/Request.php
@@ -1590,7 +1590,9 @@ public function isNoCache()
      * Gets the preferred format for the response by inspecting, in the following order:
      *   * the request format set using setRequestFormat
      *   * the values of the Accept HTTP header
-     *   * the content type of the body of the request.
+     *
+     * Note that if you use this method, you should send the "Vary: Accept" header
+     * in the response to prevent any issues with intermediary HTTP caches.
      */
     public function getPreferredFormat(?string $default = 'html'): ?string
     {
diff --git a/Response.php b/Response.php
index 714109d53..b9177934a 100644
--- a/Response.php
+++ b/Response.php
@@ -275,7 +275,7 @@ public function prepare(Request $request)
         } else {
             // Content-type based on the Request
             if (!$headers->has('Content-Type')) {
-                $format = $request->getPreferredFormat(null);
+                $format = $request->getRequestFormat(null);
                 if (null !== $format && $mimeType = $request->getMimeType($format)) {
                     $headers->set('Content-Type', $mimeType);
                 }
diff --git a/Tests/ResponseTest.php b/Tests/ResponseTest.php
index d5da59ad7..c766f88a2 100644
--- a/Tests/ResponseTest.php
+++ b/Tests/ResponseTest.php
@@ -497,12 +497,25 @@ public function testPrepareDoesNothingIfRequestFormatIsNotDefined()
         $this->assertEquals('text/html; charset=UTF-8', $response->headers->get('content-type'));
     }
 
+    /**
+     * Same URL cannot produce different Content-Type based on the value of the Accept header,
+     * unless explicitly stated in the response object.
+     */
+    public function testPrepareDoesNotSetContentTypeBasedOnRequestAcceptHeader()
+    {
+        $response = new Response('foo');
+        $request = Request::create('/');
+        $request->headers->set('Accept', 'application/json');
+        $response->prepare($request);
+
+        $this->assertSame('text/html; charset=UTF-8', $response->headers->get('content-type'));
+    }
+
     public function testPrepareSetContentType()
     {
         $response = new Response('foo');
         $request = Request::create('/');
         $request->setRequestFormat('json');
-        $request->headers->remove('accept');
 
         $response->prepare($request);