diff --git a/src/Providers/RequestXmlServiceProvider.php b/src/Providers/RequestXmlServiceProvider.php
index cb232cd..d9edb43 100644
--- a/src/Providers/RequestXmlServiceProvider.php
+++ b/src/Providers/RequestXmlServiceProvider.php
@@ -41,7 +41,7 @@ public function register()
     protected function registerWantsXml()
     {
         Request::macro('wantsXml', function () {
-            return Str::contains($this->header('Accept'), 'xml');
+            return Str::contains($this->header('Accept') ?? '', 'xml');
         });
     }