404 error on static directory

I am setting my static folder to assets, but martini is giving me a 404 error

[martini] Started GET /assets/css/main.css for [::1]:52990
[martini] Completed 404 Not Found in 325.927µs

directory structure

assets/
  |
  |__ css/
         |__ main.css

templates/
  |
  |__index.tmpl
  |__layout.tmpl

main.go

main.go

package main

import (
  "github.com/go-martini/martini"
  "github.com/martini-contrib/render"
)

func main() {
  m := martini.Classic()

  m.Use(render.Renderer(render.Options{
    Layout: "layout",
  }))

  m.Use(martini.Static("assets"))

  m.Get("/", func(r render.Render) {
    r.HTML(200, "index", nil)
  })

  m.Run()
}

layout.tmpl

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title></title>
  <link rel="stylesheet" href="assets/css/main.css">
</head>
<body>
  {{yield}}
</body>
</html>

martini-contrib / render

404 error on static directory #49