diff --git a/actionpack/lib/action_controller/metal/renderers.rb b/actionpack/lib/action_controller/metal/renderers.rb
index 67cf08445dbfe15b3896bca460f8d5802ff43670..d6f6ab1855a159b44cb6d58ff0fdbe8aa784fbe3 100644
--- a/actionpack/lib/action_controller/metal/renderers.rb
+++ b/actionpack/lib/action_controller/metal/renderers.rb
@@ -55,7 +55,7 @@ module All
     end
 
     add :json do |json, options|
-      json = json.to_json(options) unless json.respond_to?(:to_str)
+      json = json.to_json(options) unless json.kind_of?(String)
       json = "#{options[:callback]}(#{json})" unless options[:callback].blank?
       self.content_type ||= Mime::JSON
       self.response_body  = json
diff --git a/actionpack/test/controller/mime_responds_test.rb b/actionpack/test/controller/mime_responds_test.rb
index 8315f90fee993c2acccb60c0a8557e836399d93e..82969b2979d497b95eed626427de2baeee08c71e 100644
--- a/actionpack/test/controller/mime_responds_test.rb
+++ b/actionpack/test/controller/mime_responds_test.rb
@@ -599,6 +599,17 @@ def index
   end
 end
 
+class RenderJsonRespondWithController < RespondWithController
+  clear_respond_to
+  respond_to :json
+
+  def index
+    respond_with(resource) do |format|
+      format.json { render :json => RenderJsonTestException.new('boom') }
+    end
+  end
+end
+
 class EmptyRespondWithController < ActionController::Base
   def index
     respond_with(Customer.new("david", 13))
@@ -914,6 +925,13 @@ def test_block_inside_respond_with_is_rendered
     assert_equal "JSON", @response.body
   end
 
+  def test_render_json_object_responds_to_str_still_produce_json
+    @controller = RenderJsonRespondWithController.new
+    @request.accept = "application/json"
+    get :index, :format => :json
+    assert_equal %Q{{"message":"boom","error":"RenderJsonTestException"}}, @response.body
+  end
+
   def test_no_double_render_is_raised
     @request.accept = "text/html"
     assert_raise ActionView::MissingTemplate do
diff --git a/actionpack/test/lib/controller/fake_models.rb b/actionpack/test/lib/controller/fake_models.rb
index ae0c38184dc0e9c3d5d4549b0084c45327f3edf7..bd18cdc1b8ae9af17ce20d22d26d2bfb73d6fd0c 100644
--- a/actionpack/test/lib/controller/fake_models.rb
+++ b/actionpack/test/lib/controller/fake_models.rb
@@ -194,3 +194,9 @@ def each
     a.each { |i| yield i }
   end
 end
+
+class RenderJsonTestException < Exception
+  def to_json(options = nil)
+    return { :error => self.class.name, :message => self.to_str }.to_json
+  end
+end