Can not save a user with nil tokens attribute

[casertap]

The chain of callbacks induce my devise_token_auth makes it impossible to save a user with a nil tokens.

NoMethodError: undefined method `delete_if' for nil:NilClass
        from /Users/pierrecaserta/.rvm/gems/ruby-2.2.1/gems/devise_token_auth-0.1.32.beta9/app/models/devise_token_auth/concerns/user.rb:241:in `destroy_expired_tokens'
        from /Users/pierrecaserta/.rvm/gems/ruby-2.2.1/gems/activesupport-4.2.1/lib/active_support/callbacks.rb:432:in `block in make_lambda'
        from /Users/pierrecaserta/.rvm/gems/ruby-2.2.1/gems/activesupport-4.2.1/lib/active_support/callbacks.rb:164:in `call'
        from /Users/pierrecaserta/.rvm/gems/ruby-2.2.1/gems/activesupport-4.2.1/lib/active_support/callbacks.rb:164:in `block in halting'
        from /Users/pierrecaserta/.rvm/gems/ruby-2.2.1/gems/activesupport-4.2.1/lib/active_support/callbacks.rb:504:in `call'
        from /Users/pierrecaserta/.rvm/gems/ruby-2.2.1/gems/activesupport-4.2.1/lib/active_support/callbacks.rb:504:in `block in call'
        from /Users/pierrecaserta/.rvm/gems/ruby-2.2.1/gems/activesupport-4.2.1/lib/active_support/callbacks.rb:504:in `each'
        from /Users/pierrecaserta/.rvm/gems/ruby-2.2.1/gems/activesupport-4.2.1/lib/active_support/callbacks.rb:504:in `call'
        from /Users/pierrecaserta/.rvm/gems/ruby-2.2.1/gems/activesupport-4.2.1/lib/active_support/callbacks.rb:92:in `_run_callbacks'
        from /Users/pierrecaserta/.rvm/gems/ruby-2.2.1/gems/activesupport-4.2.1/lib/active_support/callbacks.rb:776:in `_run_save_callbacks'
        from /Users/pierrecaserta/.rvm/gems/ruby-2.2.1/gems/activerecord-4.2.1/lib/active_record/callbacks.rb:302:in `create_or_update'
        from /Users/pierrecaserta/.rvm/gems/ruby-2.2.1/gems/activerecord-4.2.1/lib/active_record/persistence.rb:142:in `save!'
        from /Users/pierrecaserta/.rvm/gems/ruby-2.2.1/gems/activerecord-4.2.1/lib/active_record/validations.rb:43:in `save!'
        from /Users/pierrecaserta/.rvm/gems/ruby-2.2.1/gems/activerecord-4.2.1/lib/active_record/attribute_methods/dirty.rb:29:in `save!'
        from /Users/pierrecaserta/.rvm/gems/ruby-2.2.1/gems/activerecord-4.2.1/lib/active_record/transactions.rb:291:in `block in save!'