diff --git a/system/src/Grav/Common/Errors/Errors.php b/system/src/Grav/Common/Errors/Errors.php
index 688068451..00be73811 100644
--- a/system/src/Grav/Common/Errors/Errors.php
+++ b/system/src/Grav/Common/Errors/Errors.php
@@ -14,7 +14,7 @@ class Errors
     {
         $grav = Grav::instance();
         $config = $grav['config']->get('system.errors');
-        $jsonRequest = $_SERVER && $_SERVER['HTTP_ACCEPT'] && $_SERVER['HTTP_ACCEPT'] == 'application/json';
+        $jsonRequest = $_SERVER && isset($_SERVER['HTTP_ACCEPT']) && $_SERVER['HTTP_ACCEPT'] == 'application/json';
 
         // Setup Whoops-based error handler
         $whoops = new \Whoops\Run;