diff --git a/actionpack/lib/action_dispatch/middleware/debug_locks.rb b/actionpack/lib/action_dispatch/middleware/debug_locks.rb
index 323bc9bd71..831e5e248c 100644
--- a/actionpack/lib/action_dispatch/middleware/debug_locks.rb
+++ b/actionpack/lib/action_dispatch/middleware/debug_locks.rb
@@ -99,8 +99,8 @@ def render_details(req)
           msg << "\n#{info[:backtrace].join("\n")}\n" if info[:backtrace]
         end.join("\n\n---\n\n\n")
 
-        [200, { "Content-Type" => "text/plain; charset=#{ActionDispatch::Response.default_charset}",
-          "Content-Length" => str.size }, [str]]
+        [200, { Rack::CONTENT_TYPE => "text/plain; charset=#{ActionDispatch::Response.default_charset}",
+                Rack::CONTENT_LENGTH => str.size.to_s }, [str]]
       end
 
       def blocked_by?(victim, blocker, all_threads)
diff --git a/actionpack/test/dispatch/debug_locks_test.rb b/actionpack/test/dispatch/debug_locks_test.rb
index d69614bd79..565e83071b 100644
--- a/actionpack/test/dispatch/debug_locks_test.rb
+++ b/actionpack/test/dispatch/debug_locks_test.rb
@@ -32,7 +32,9 @@ def test_render_threads_status
   private
     def build_app
       @app = self.class.build_app do |middleware|
+        middleware.use Rack::Lint
         middleware.use ActionDispatch::DebugLocks
+        middleware.use Rack::Lint
       end
     end
 end