terminal error

Started GET "/posts/11/edit" for 127.0.0.1 at 2015-04-04 22:41:21 +0200
Processing by PostsController#edit as HTML
  Parameters: {"id"=>"11"}
  User Load (1.6ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ?  ORDER BY "users"."id" ASC LIMIT 1  [["id", 27]]
  Post Load (0.1ms)  SELECT  "posts".* FROM "posts" WHERE "posts"."id" = ? LIMIT 1  [["id", 11]]
  Rendered posts/_form.html.erb (5.8ms)
  Rendered posts/edit.html.erb within layouts/application (6.7ms)
  Rendered layouts/_head_tag.html.erb (242.7ms)
  Rendered layouts/_header.html.erb (5.1ms)
  Rendered layouts/_messages.html.erb (0.3ms)
  Rendered layouts/_footer.html.erb (0.3ms)
Completed 200 OK in 266ms (Views: 260.4ms | ActiveRecord: 1.7ms)


Started PATCH "/posts/11" for 127.0.0.1 at 2015-04-04 22:41:22 +0200
Processing by PostsController#update as HTML
  Parameters: {"utf8"=>"✓", "authenticity_token"=>"NGM3j9b3ZylUWa7cGnW9hhCj0JhAnEYj/3FVqXSeKwnrIpIEUg3YG7aRTvT63+wh/0iigndacpvpvChIt817/w==", "post"=>{"title"=>"تجربة 1", "content_md"=>"*تجربة*\r\n\r\n_تجربة_", "post_type"=>"1"}, "commit"=>"أرسِل", "id"=>"11"}
  User Load (0.1ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ?  ORDER BY "users"."id" ASC LIMIT 1  [["id", 27]]
  Post Load (0.1ms)  SELECT  "posts".* FROM "posts" WHERE "posts"."id" = ? LIMIT 1  [["id", 11]]
Completed 500 Internal Server Error in 5ms

TypeError (wrong argument type nil (expected String)):
  app/controllers/posts_controller.rb:61:in `render'
  app/controllers/posts_controller.rb:61:in `update'


Processing by ErrorsController#internal_server_error as HTML
  Parameters: {"utf8"=>"✓", "authenticity_token"=>"NGM3j9b3ZylUWa7cGnW9hhCj0JhAnEYj/3FVqXSeKwnrIpIEUg3YG7aRTvT63+wh/0iigndacpvpvChIt817/w==", "post"=>{"title"=>"تجربة 1", "content_md"=>"*تجربة*\r\n\r\n_تجربة_", "post_type"=>"1"}, "commit"=>"أرسِل", "id"=>"11"}
  Rendered errors/internal_server_error.html.erb within layouts/application (0.3ms)
  Rendered layouts/_head_tag.html.erb (297.6ms)
  Rendered layouts/_header.html.erb (6.3ms)
  Rendered layouts/_messages.html.erb (0.3ms)
  Rendered layouts/_footer.html.erb (0.3ms)
Completed 200 OK in 318ms (Views: 315.5ms | ActiveRecord: 0.0ms)