mailboxer error


I want to make my user model act as messageable.
I added gem 'mailboxer', github: 'mailboxer/mailboxer' in the gem file.
I added 'acts_as_messageable' in the user model.

I assiged u1 = User.first
u2 = User.find_by(id: 2)
when I write the following command in console:
u1.send_message u2, "body", "subject"

I get the error:
NoMethodError: undefined method `email' for

Does anybody knows what can be the problem?
Thank you!

Did you follow all the instructions here:

Also why are you installing direct from github? Doing that gives you
the latest development version rather than the released version.