TypeError nil is not a symbol or string

gistfile1.rb

my_table = "table_name"
conn = ActiveRecord::Base.connection

begin
  conn.execute "CREATE TABLE #{my_table} (key INTEGER, val TEXT);"

  klass = Class.new(ActiveRecord::Base) do
          self.table_name = my_table
          primary_key = "key"
          default_scope { order("key DESC NULLS LAST") }
  end

  tmp = klass.new({key: 1})
  tmp.val = "hello"
  tmp.save

  tmp = klass.where({key: 1}).first
  tmp.val = "goodbye"
  tmp.save

ensure
  conn.execute "DROP TABLE #{my_table}"
end

# 2.2.2 :062 > load 'test.rb'
# TypeError: nil is not a symbol nor a string
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activemodel-4.2.1/lib/active_model/dirty.rb:181:in `attribute_was'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activerecord-4.2.1/lib/active_record/attribute_methods/primary_key.rb:45:in `id_was'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activerecord-4.2.1/lib/active_record/persistence.rb:513:in `_update_record'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activerecord-4.2.1/lib/active_record/locking/optimistic.rb:79:in `_update_record'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activerecord-4.2.1/lib/active_record/attribute_methods/dirty.rb:127:in `_update_record'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activerecord-4.2.1/lib/active_record/callbacks.rb:310:in `block in _update_record'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activesupport-4.2.1/lib/active_support/callbacks.rb:88:in `call'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activesupport-4.2.1/lib/active_support/callbacks.rb:88:in `_run_callbacks'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activesupport-4.2.1/lib/active_support/callbacks.rb:776:in `_run_update_callbacks'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activerecord-4.2.1/lib/active_record/callbacks.rb:310:in `_update_record'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activerecord-4.2.1/lib/active_record/timestamp.rb:70:in `_update_record'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activerecord-4.2.1/lib/active_record/persistence.rb:502:in `create_or_update'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activerecord-4.2.1/lib/active_record/callbacks.rb:302:in `block in create_or_update'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activesupport-4.2.1/lib/active_support/callbacks.rb:88:in `call'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activesupport-4.2.1/lib/active_support/callbacks.rb:88:in `_run_callbacks'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activesupport-4.2.1/lib/active_support/callbacks.rb:776:in `_run_save_callbacks'
# ... 5 levels...
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activerecord-4.2.1/lib/active_record/transactions.rb:347:in `block in with_transaction_returning_status'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activerecord-4.2.1/lib/active_record/connection_adapters/abstract/database_statements.rb:213:in `block in transaction'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activerecord-4.2.1/lib/active_record/connection_adapters/abstract/transaction.rb:188:in `within_new_transaction'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activerecord-4.2.1/lib/active_record/connection_adapters/abstract/database_statements.rb:213:in `transaction'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activerecord-4.2.1/lib/active_record/transactions.rb:220:in `transaction'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activerecord-4.2.1/lib/active_record/transactions.rb:344:in `with_transaction_returning_status'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activerecord-4.2.1/lib/active_record/transactions.rb:286:in `block in save'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activerecord-4.2.1/lib/active_record/transactions.rb:301:in `rollback_active_record_state!'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activerecord-4.2.1/lib/active_record/transactions.rb:285:in `save'
#         from test.rb:19:in `<top (required)>'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:268:in `load'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:268:in `block in load'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:240:in `load_dependency'
#         from /Users/tramfjord/.rvm/gems/ruby-2.2.2/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:268:in `load'
#         from (irb):62