diff --git a/activerecord/lib/active_record/middleware/database_selector.rb b/activerecord/lib/active_record/middleware/database_selector.rb
index b5b5df074c..93a1a39c3e 100644
--- a/activerecord/lib/active_record/middleware/database_selector.rb
+++ b/activerecord/lib/active_record/middleware/database_selector.rb
@@ -35,10 +35,10 @@ module ActiveRecord
     #   config.active_record.database_resolver = MyResolver
     #   config.active_record.database_resolver_context = MyResolver::MySession
     class DatabaseSelector
-      def initialize(app, resolver_klass = Resolver, context_klass = Resolver::Session, options = {})
+      def initialize(app, resolver_klass = nil, context_klass = nil, options = {})
         @app = app
-        @resolver_klass = resolver_klass
-        @context_klass = context_klass
+        @resolver_klass = resolver_klass || Resolver
+        @context_klass = context_klass || Resolver::Session
         @options = options
       end