wrong number of arguments (2 for 1), UsersController in Rails

I'm a Rails newbie making my way through Michael Hartl's Ruby on Rails Tutorial and, while I have been able to troubleshoot a few errors on my own, this one from my trek through chapter 9 is sending me in circles.

(What I assume are) the relevant parts of the Users controller:

class UsersController < ApplicationController
  before_action :logged_in_user, only: [:index, :edit, :update]
  before_action :correct_user,   only: [:edit, :update]

  def index
    @users = User.all
  end

My views/users/index.html.erb:

<% provide(:title, 'All users') %>
<h1>All users</h1>

<ul class="users">
  <% @users.each do |user| %> # Line flagged with error
    <li>
      <%= gravatar_for user, size: 50 %>
      <%= link_to user.name, user %>
    </li>
  <% end %>
</ul>

My Users helper:

module UsersHelper

def gravatar_for(user) # line flagged with error
    gravatar_id = Digest::MD5::hexdigest(user.email.downcase)
    gravatar_url = "http://ift.tt/1d642xH}"
    image_tag(gravatar_url, alt: user.name, class: "gravatar")
  end
end

The error also flags another line of code that doesn't exist in my files; I guess it comes from the Ruby to HTML conversion?

delegate :to_xml, :to_yaml, :length, :collect, :map, :each, :all?, :include?, :to_ary, :join, to: :to_a

I've been scratching my head at this for almost two hours with no progress. All I know is that these errors go away when I replace the index page with simple text. Is this a problem with Gravatar? I really have no clue and would appreciate any help.