变更集中的参数错误

我正在从本书中学习Phoenix框架，Programming Phoenix并陷入了我似乎无法理解的错误。基本上，这本书目前是在添加用于创建新用户的表单的时候。因此，当我将浏览器指向时，http://localhost:4000/users/new我得到以下页面（请展开图像）：

在命令行输出中，服务器显示：

[info] GET /users/new
[debug] Processing by Rumbl.UserController.new/2
  Parameters: %{}
  Pipelines: [:browser]
[info] Sent 500 in 9ms
[error] #PID<0.543.0> running Rumbl.Endpoint terminated
Server: localhost:4000 (http)
Request: GET /users/new
** (exit) an exception was raised:
    ** (ArgumentError) argument error
        :erlang.binary_to_existing_atom("name,", :utf8)
        (ecto) lib/ecto/changeset.ex:402: Ecto.Changeset.process_empty_fields/2
        (elixir) lib/enum.ex:1184: Enum."-map/2-lists^map/1-0-"/2
        (ecto) lib/ecto/changeset.ex:373: Ecto.Changeset.do_cast/7
        (rumbl) web/models/user.ex:15: Rumbl.User.changeset/2
        (rumbl) web/controllers/user_controller.ex:17: Rumbl.UserController.new/2
        (rumbl) web/controllers/user_controller.ex:1: Rumbl.UserController.action/2
        (rumbl) web/controllers/user_controller.ex:1: Rumbl.UserController.phoenix_controller_pipeline/2
        (rumbl) lib/rumbl/endpoint.ex:1: Rumbl.Endpoint.instrument/4
        (rumbl) lib/phoenix/router.ex:261: Rumbl.Router.dispatch/2
        (rumbl) web/router.ex:1: Rumbl.Router.do_call/2
        (rumbl) lib/rumbl/endpoint.ex:1: Rumbl.Endpoint.phoenix_pipeline/1
        (rumbl) lib/plug/debugger.ex:122: Rumbl.Endpoint."call (overridable 3)"/2
        (rumbl) lib/rumbl/endpoint.ex:1: Rumbl.Endpoint.call/2
        (plug) lib/plug/adapters/cowboy/handler.ex:15: Plug.Adapters.Cowboy.Handler.upgrade/4
        (cowboy) src/cowboy_protocol.erl:442: :cowboy_protocol.execute/4

由于错误表明binary_to_existing_atom我想可能是我输入了错误的参数或类似的东西，但事实并非如此。

我的模型文件：

defmodule Rumbl.User do
    use Rumbl.Web, :model

    schema "users" do
        field :name, :string
        field :username, :string
        field :password, :string, virtual: true
        field :password_hash, :string

        timestamps
    end

    def changeset(model, params \\ :invalid) do
        model
        |> cast(params, ~w(name, username), [])
        |> validate_length(:username, min: 1, max: 20)
    end 
end

我的迁移文件：

defmodule Rumbl.Repo.Migrations.CreateUser do
    use Ecto.Migration

    def change do
        create table(:users) do
            add :name, :string
            add :username, :string, null: false
            add :password_hash, :string

            timestamps
        end

        create unique_index(:users, [:username])
    end
end

控制器：

defmodule Rumbl.UserController do
    use Rumbl.Web, :controller

    def index(conn, _params) do
        users = Repo.all(Rumbl.User)
        render conn, "index.html", users: users
    end

    def show(conn, %{"id" => id}) do
        user = Repo.get(Rumbl.User, id)
        render conn, "show.html", user: user
    end

    alias Rumbl.User

    def new(conn, _params) do
        changeset = User.changeset(%User{})
        render conn, "new.html", changeset: changeset 
    end
end

最后，模板：

<h1>New User</h1>

<%= form_for @changeset, user_path(@conn, :create), fn f -> %>
    <div class="form-group">
        <%= text_input f, :name, placeholder: "Username", class: "form-control" %>        
    </div>
    <div class="form-group">
        <%= password_input f, :password, placeholder: "Password", class: "form-control" %>        
    </div>
    <%= submit "Create User", class: "btn btn-primary" %>
<% end %>

让我知道是否还需要其他代码。并在此先感谢！