Railsチュートリアル6章で$rails test実行時に発生したエラーを解決したい

前提・実現したいこと

Railsチュートリアル６章をやっており、6.34終了時(Railsチュートリアル6章)に$rails testを実行したら、
エラーが発生してしまいました。ググってみたり自分なりにいろいろ試したのですが、解決できませんでした。原因究明して先に進みたいので、どなたかご教授いただけると幸いです。

発生している問題・エラーメッセージ

Running via Spring preloader in process 11946
Started with run options --seed 47576

Run options: --seed 47576--=---=---=---=---=---=---=---=---=---=---=---=---=---=---=---=---=---=---=---=---=---=-] 0% Time: 00:00:00,  ETA: ??:??:??

# Running:

.  16/2: [============                                                                                           ] 12% Time: 00:00:00,  ETA: 00:00:0..  
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16/6: [======================================                                                                 ] 37% Time: 00:00:00,  ETA: 00:00:.  
16/7: [============================================                                                           ] 43% Time: 00:00:00,  ETA: 00:00:0.....  
16/12: [============================================================================                          ] 75% Time: 00:00:00,  ETA: 00: 
FAIL["test_email_addresses_should_be_unique", UserTest, 0.41908737100129656]
 test_email_addresses_should_be_unique#UserTest (0.42s)
        Expected true to be nil or false
        test/models/user_test.rb:53:in `block in <class:UserTest>'

F 16/13: [==================================================================================                    ] 81% Time: 00:00:00,  ETA: 00:00:00

Failure:
UserTest#test_email_addresses_should_be_unique [/home/ec2-user/environment/sample_app/test/models/user_test.rb:53]:
Expected true to be nil or false


bin/rails test test/models/user_test.rb:49

.  16/16: [=====================================================================================================] 100% Time: 00:00:00, Time: 00:00:00
.
Finished in 0.43870s
16 tests, 36 assertions, 1 failures, 0 errors, 0 skips



Finished in 0.450738s, 35.4974 runs/s, 79.8691 assertions/s.
16 runs, 36 assertions, 1 failures, 0 errors, 0 skips

該当のソースコード

require 'test_helper'

class UserTest < ActiveSupport::TestCase
  
  def setup
    @user = User.new(name: "Example User", email: "user@example.com")
  end
  
  test "should be valid" do
    assert @user.valid?
  end
  
  test "name should be present" do
    @user.name = ""
    assert_not @user.valid?
  end
  
  test "email should be present" do
    @user.email = ""
    assert_not @user.valid?
  end
  
  test "name should not be too long" do
    @user.name = "a"*51
    assert_not @user.valid?
  end
  
  test "email should not be too long" do
    @user.email = "a"*244 + "@example.com"
    assert_not @user.valid?
  end
  
  test "email validation should accept valid addresses" do
    valid_addresses = %w[user@example.com USER@foo.COM A_US-ER@foo.bar.org first.last@foo.jp alice+bob@baz.cn]
    valid_addresses.each do |valid_address|
      @user.email = valid_address
      assert @user.valid?, "#{valid_address.inspect} should be valid"
    end
  end
  
  test "email validation should reject invalid addresses" do
    invalid_addresses = %w[user@example,com user_at_foo.org user.name@example. foo@bar_baz.com foo@bar+baz.com foo@bar..com]
    invalid_addresses.each do |invalid_address|
      @user.email = invalid_address
      assert_not @user.valid?, "#{invalid_address.inspect} should be invalid"
    end
  end
  
  test "email addresses should be unique" do
    duplicate_user = @user.dup
    duplicate_user.email = @user.email.upcase
    @user.save
    assert_not duplicate_user.valid?
  end
  
  test "email address should be saved as a lower-case" do
    mixed_case_email = "Foo@ExAMPle.CoM"
    @user.email = mixed_case_email
    @user.save
    assert_equal mixed_case_email.downcase, @user.reload.email
  end
end

class User < ApplicationRecord
  before_save { self.email = email.downcase! }
  validates :name, presence: true, length: { maximum:50 }
  VALID_EMAIL_REGEX = /\A[\w+\-.]+@[a-z\d\-]+(.[a-z\d\-]+)*.[a-z]+\z/i
  validates :email, presence: true, length: { maximum: 255 }, 
            format: { with: VALID_EMAIL_REGEX },
            uniqueness: { case_sensitive: false }
end