Check if the routes object really responds to define_mount_prefix.

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -327,7 +327,7 @@ def app_name(app)
           end
 
           def define_generate_prefix(app, name)
-            return unless app.respond_to?(:routes)
+            return unless app.respond_to?(:routes) && app.routes.respond_to?(:define_mounted_helper)
 
             _route = @set.named_routes.routes[name.to_sym]
             _routes = @set

actionpack/test/dispatch/mount_test.rb

@@ -2,6 +2,17 @@
 
 class TestRoutingMount < ActionDispatch::IntegrationTest
   Router = ActionDispatch::Routing::RouteSet.new
+
+  class FakeEngine
+    def self.routes
+      Object.new
+    end
+
+    def self.call(env)
+      [200, {"Content-Type" => "text/html"}, ["OK"]]
+    end
+  end
+
   Router.draw do
     SprocketsApp = lambda { |env|
       [200, {"Content-Type" => "text/html"}, ["#{env["SCRIPT_NAME"]} -- #{env["PATH_INFO"]}"]]
@@ -10,6 +21,8 @@ class TestRoutingMount < ActionDispatch::IntegrationTest
     mount SprocketsApp, :at => "/sprockets"
     mount SprocketsApp => "/shorthand"
 
+    mount FakeEngine, :at => "/fakeengine"
+
     scope "/its_a" do
       mount SprocketsApp, :at => "/sprocket"
     end
@@ -33,4 +46,9 @@ def test_mounting_with_shorthand
     get "/shorthand/omg"
     assert_equal "/shorthand -- /omg", response.body
   end
+
+  def test_with_fake_engine_does_not_call_invalid_method
+    get "/fakeengine"
+    assert_equal "OK", response.body
+  end
 end