diff --git a/app.rb b/app.rb
index b7d14b7..7bd129b 100644
--- a/app.rb
+++ b/app.rb
@@ -16,6 +16,13 @@ module Maptool
     set :public_folder, File.join(__dir__, "public")
     set :views, File.join(__dir__, "views")
 
+    # sinatra/json defaults to MultiJson, whose `encode` (what
+    # sinatra-contrib 4.2.1 actually calls) is deprecated in newer
+    # multi_json versions in favor of a constant this app has no other use
+    # for — stdlib JSON already does the same job and every use of the
+    # `json` helper below is a plain hash/array, nothing MultiJson-specific.
+    set :json_encoder, JSON
+
     # A persistent secret across restarts/redeploys — otherwise every deploy
     # would invalidate every logged-in session's cookie (see
     # docs/deployment.adoc). Falls back to a random one so the app still