undefined method `default' for Class

When using 'save' on an existing and previously saved document, there's a 'undefined method'-error on 'default'. 'save' and 'update' result in the same error.

class TestDoc < Couchbase::Model
attribute :email, :default => ''
attribute :created_at, :default => lambda

view :by_created_at
end

t = TestDoc.new(:id => request.session_options[:id])
t.save
t = TestDoc.find(request.session_options[:id])
t.email = 'hello'
t.update(:email => 'blubb')

undefined method `default' for TestDoc(id, email, created_at):Class

couchbase-model (0.4.0) lib/couchbase/model.rb:487:in `save'
(eval):4:in `block in save'
activesupport (3.2.8) lib/active_support/callbacks.rb:403:in `run3218406879367123135save4511417429032474671_callbacks'
activesupport (3.2.8) lib/active_support/callbacks.rb:405:in `__run_callback'
activesupport (3.2.8) lib/active_support/callbacks.rb:385:in `_run_save_callbacks'
activesupport (3.2.8) lib/active_support/callbacks.rb:81:in `run_callbacks'
(eval):3:in `save'
couchbase-model (0.4.0) lib/couchbase/model.rb:501:in `update'
(eval):4:in `block in update'
activesupport (3.2.8) lib/active_support/callbacks.rb:403:in `run3218406879367123135update4511417429032474671_callbacks'
activesupport (3.2.8) lib/active_support/callbacks.rb:405:in `__run_callback'
activesupport (3.2.8) lib/active_support/callbacks.rb:385:in `_run_update_callbacks'
activesupport (3.2.8) lib/active_support/callbacks.rb:81:in `run_callbacks'
(eval):3:in `update'