try adding permalink to 404 page

Author: daattali

404.html

@@ -1,6 +1,7 @@
 ---
 layout: default
 title: 404 - Page not found
+permalink: /404.html
 ---
 
 <div class="text-center">
@@ -9,4 +10,4 @@ <h1>Move along. (404 error)</h1>
   <br/>
 
   <img src="{{ site.baseurl }}/img/404-southpark.jpg" />
-</div>
+</div>