Test fails: association id should be mandatory

I have the following integration tests:

def setup
  @file = fixture_file_upload('test/fixtures/rails.jpg')
  @user = users(:one)
  @image = @user.build_image(image_file: @file)
end

test "should be valid" do
  assert @image.valid?
end

test "image_file should be present" do
  @image.image_file = "     "
  assert_not @image.valid?
end

test "user_id should be present" do
  @image.user_id = "     "
  assert_not @image.valid?
end

The last integration test fails -- i.e., @image is valid -- which I don't understand. Given the migration file below, shouldn't user_id be mandatory?

def change
  create_table :images do |t|
    t.references :user, index: true, foreign_key: true
    t.string :image_file, null: false

    t.timestamps null: false
  end
end