Raise a helpful error message on #mount misuse

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -430,6 +430,10 @@ def mount(app, options = nil)
           if options
             path = options.delete(:at)
           else
+            unless Hash === app
+              raise ArgumentError, "must be called with mount point"
+            end
+
             options = app
             app, path = options.find { |k, v| k.respond_to?(:call) }
             options.delete(app) if app

actionpack/test/dispatch/mapper_test.rb

@@ -98,6 +98,15 @@ def test_map_wildcard_with_format_true
         mapper.get '/*path', :to => 'pages#show', :format => true
         assert_equal '/*path.:format', fakeset.conditions.first[:path_info]
       end
+
+      def test_raising_helpful_error_on_invalid_arguments
+        fakeset = FakeSet.new
+        mapper = Mapper.new fakeset
+        app = lambda { |env| [200, {}, [""]] }
+        assert_raises ArgumentError do
+          mapper.mount app
+        end
+      end
     end
   end
 end