diff --git a/library/HTMLPurifier/Lexer.php b/library/HTMLPurifier/Lexer.php
index 99b3c7df..e9da3ed5 100644
--- a/library/HTMLPurifier/Lexer.php
+++ b/library/HTMLPurifier/Lexer.php
@@ -96,7 +96,7 @@ class HTMLPurifier_Lexer
                         break;
                     }
 
-                    if (class_exists('DOMDocument') &&
+                    if (class_exists('DOMDocument', false) &&
                         method_exists('DOMDocument', 'loadHTML') &&
                         !extension_loaded('domxml')
                     ) {