diff --git a/src/Pecee/Http/Request.php b/src/Pecee/Http/Request.php
index 63bd5d6..7f225ea 100644
--- a/src/Pecee/Http/Request.php
+++ b/src/Pecee/Http/Request.php
@@ -364,7 +364,7 @@ class Request
      */
     public function isAjax(): bool
     {
-        return (strtolower($this->getHeader('http-x-requested-with')) === 'xmlhttprequest');
+        return (strtolower((string)$this->getHeader('http-x-requested-with')) === 'xmlhttprequest');
     }
 
     /**