diff --git a/src/Message/DirectAuthorizeRequest.php b/src/Message/DirectAuthorizeRequest.php
index 036642c..ab868b2 100644
--- a/src/Message/DirectAuthorizeRequest.php
+++ b/src/Message/DirectAuthorizeRequest.php
@@ -75,8 +75,8 @@ protected function getBaseAuthorizeData()
         $data['BrowserJavascriptEnabled'] = $this->getBrowserJavascriptEnabled() ?: static::BROWSER_JAVASCRIPT_NO;
         $data['BrowserLanguage'] = $this->getBrowserLanguage() ?: static::BROWSER_LANGUAGE;
         $data['ThreeDSNotificationURL'] = $this->getThreeDSNotificationURL();
-        $data['BrowserAcceptHeader'] = $_SERVER['HTTP_ACCEPT'];
-        $data['BrowserUserAgent'] = $_SERVER['HTTP_USER_AGENT'];
+        $data['BrowserAcceptHeader'] = !empty($_SERVER['HTTP_ACCEPT']) ? $_SERVER['HTTP_ACCEPT'] : null;
+        $data['BrowserUserAgent'] = !empty($_SERVER['HTTP_USER_AGENT']) ? $_SERVER['HTTP_USER_AGENT'] : null;
         $data['ChallengeWindowSize'] = $this->getChallengeWindowSize() ?: static::CHALLENGE_WINDOW_SIZE_05;
         // ----
         // ---- "4.00" - required if BrowserJavascriptEnabled == "1"