diff --git a/library/Icinga/Web/Response.php b/library/Icinga/Web/Response.php
index 2a90a97b1..ac2573f8d 100644
--- a/library/Icinga/Web/Response.php
+++ b/library/Icinga/Web/Response.php
@@ -300,7 +300,9 @@ class Response extends Zend_Controller_Response_Http
             }
         }
 
-        $this->setHeader('Content-Type', $this->getContentType(), false);
+        if (!$this->getHeader('Content-Type', true)) {
+            $this->setHeader('Content-Type', $this->getContentType());
+        }
     }
 
     /**