2 files changed, 17 insertions, 4 deletions
diff --git a/actionpack/lib/action_dispatch/routing/mapper.rb b/actionpack/lib/action_dispatch/routing/mapper.rb
index 3f7cf0950d..b618b9c400 100644
--- a/actionpack/lib/action_dispatch/routing/mapper.rb
+++ b/actionpack/lib/action_dispatch/routing/mapper.rb
@@ -664,7 +664,6 @@ module ActionDispatch
           def define_generate_prefix(app, name)
             _route = @set.named_routes.get name
             _routes = @set
-            _url_helpers = @set.url_helpers
 
             script_namer = ->(options) do
               prefix_options = options.slice(*_route.segment_keys)
@@ -676,7 +675,7 @@ module ActionDispatch
 
               # We must actually delete prefix segment keys to avoid passing them to next url_for.
               _route.segment_keys.each { |k| options.delete(k) }
-              _url_helpers.send("#{name}_path", prefix_options)
+              @set.url_helpers.send("#{name}_path", prefix_options)
             end
 
             app.routes.define_mounted_helper(name, script_namer)
diff --git a/actionpack/lib/action_dispatch/routing/route_set.rb b/actionpack/lib/action_dispatch/routing/route_set.rb
index acce8a7ef3..8f21722a6a 100644
--- a/actionpack/lib/action_dispatch/routing/route_set.rb
+++ b/actionpack/lib/action_dispatch/routing/route_set.rb
@@ -378,6 +378,7 @@ module ActionDispatch
         @disable_clear_and_finalize = false
         @finalized                  = false
         @env_key                    = "ROUTES_#{object_id}_SCRIPT_NAME".freeze
+        @url_helpers                = nil
 
         @set    = Journey::Routes.new
         @router = Journey::Router.new @set
@@ -437,6 +438,7 @@ module ActionDispatch
         return if @finalized
         @append.each { |blk| eval_block(blk) }
         @finalized = true
+        @url_helpers = build_url_helper_module true
       end
 
       def clear!
@@ -465,11 +467,10 @@ module ActionDispatch
         return if MountedHelpers.method_defined?(name)
 
         routes = self
-        helpers = routes.url_helpers
 
         MountedHelpers.class_eval do
           define_method "_#{name}" do
-            RoutesProxy.new(routes, _routes_context, helpers, script_namer)
+            RoutesProxy.new(routes, _routes_context, routes.url_helpers, script_namer)
           end
         end
 
@@ -480,7 +481,20 @@ module ActionDispatch
         RUBY
       end
 
+      class UnfinalizedRouteSet < StandardError
+      end
+
       def url_helpers(supports_path = true)
+        raise UnfinalizedRouteSet, "routes have not been finalized. Please call `finalize!` or use `draw(&block)`" unless @finalized
+
+        if supports_path
+          @url_helpers
+        else
+          build_url_helper_module false
+        end
+      end
+
+      def build_url_helper_module(supports_path)
         routes = self
 
         Module.new do