diff --git a/lib/rubygems/server.rb b/lib/rubygems/server.rb
index f8a653b..b46224d 100644
--- a/lib/rubygems/server.rb
+++ b/lib/rubygems/server.rb
@@ -809,7 +809,8 @@ def run
       '/gems' => '/cache/',
     }
 
-    @server.mount '/doc_root', RDoc::Servlet, '/doc_root'
+    servlet = defined?(RDoc::Servlet) ? RDoc::Servlet : RDoc::RI::Servlet
+    @server.mount '/doc_root', servlet, '/doc_root'
 
     @gem_dirs.each do |gem_dir|
       file_handlers.each do |mount_point, mount_dir|