PG::InvalidTextRepresentation exception

bugreport.rb

# Activate the gem you are reporting the issue against.
gem 'activerecord', '4.2.4'
require 'active_record'
require 'minitest/autorun'
require 'logger'

# Ensure backward compatibility with Minitest 4
Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: 'postgresql', host: 'postgresql.service.consul', database: 'bugreport', username: 'postgres', password: '')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  if execute("SELECT EXISTS (SELECT 1 FROM pg_type WHERE typname = 'post_types')").to_a.first["exists"] == "f"
    execute "CREATE TYPE post_types AS ENUM ('some_type')"
  end

  create_table :posts, force: true do |t|
    t.column :post_type, :post_types, null: true
  end
end

class Post < ActiveRecord::Base
end


class BugTest < Minitest::Test
  def test_enum_null
    # No error, `post_type` can be nil
    comment = Post.create!
    assert_equal "", Post.find(comment.id).post_type

    # No error, valid value for `post_type`
    comment.update_attribute(:post_type, "some_type")
    assert_equal "some_type", Post.find(comment.id).post_type

    # PG error, because of string typecast
    comment.update_attribute(:post_type, nil)
    assert_equal nil, Post.find(comment.id).post_type
  end
end