diff --git a/packages/forest_admin_rpc_agent/lib/forest_admin_rpc_agent/routes/base_route.rb b/packages/forest_admin_rpc_agent/lib/forest_admin_rpc_agent/routes/base_route.rb
index b52e8b672..8e07c0651 100644
--- a/packages/forest_admin_rpc_agent/lib/forest_admin_rpc_agent/routes/base_route.rb
+++ b/packages/forest_admin_rpc_agent/lib/forest_admin_rpc_agent/routes/base_route.rb
@@ -8,10 +8,10 @@ def initialize(url, method, name)
       end
 
       def registered(app)
-        if defined?(Sinatra) && (app == Sinatra::Base || app.ancestors.include?(Sinatra::Base))
-          register_sinatra(app)
-        elsif defined?(Rails) && app.is_a?(ActionDispatch::Routing::Mapper)
+        if defined?(Rails) && app.is_a?(ActionDispatch::Routing::Mapper)
           register_rails(app)
+        elsif defined?(Sinatra) && app.is_a?(Class) && (app == Sinatra::Base || app.ancestors.include?(Sinatra::Base))
+          register_sinatra(app)
         else
           raise NotImplementedError,
                 "Unsupported application type: #{app.class}. #{self} works with Sinatra::Base or ActionDispatch::Routing::Mapper."
diff --git a/packages/forest_admin_rpc_agent/spec/lib/forest_admin_rpc_agent/routes/base_route_spec.rb b/packages/forest_admin_rpc_agent/spec/lib/forest_admin_rpc_agent/routes/base_route_spec.rb
index 1c7578647..f414f6ac6 100644
--- a/packages/forest_admin_rpc_agent/spec/lib/forest_admin_rpc_agent/routes/base_route_spec.rb
+++ b/packages/forest_admin_rpc_agent/spec/lib/forest_admin_rpc_agent/routes/base_route_spec.rb
@@ -49,6 +49,21 @@ def handle_request(_params)
           end
         end
 
+        context 'when the app is an ActionDispatch::Routing::Mapper and Sinatra is defined' do
+          let(:rails_app) { Class.new(Rails::Application) }
+          let(:rails_router) { ActionDispatch::Routing::Mapper.new(rails_app.routes) }
+
+          before do
+            stub_const('Sinatra::Base', Class.new)
+            allow(route).to receive(:register_rails)
+          end
+
+          it 'calls register_rails without raising NoMethodError' do
+            expect { route.registered(rails_router) }.not_to raise_error
+            expect(route).to have_received(:register_rails).with(rails_router)
+          end
+        end
+
         context 'when the app is neither Sinatra nor Rails' do
           let(:unknown_app) { Object.new }