in `lambda': tried to create Proc object without a block (ArgumentError)

scope :for_user, (lambda {|user_id| a = Follow.follows(user_id); Question.where{user_id.in(a.select{followed_id})}})

Gives me:

`lambda': tried to create Proc object without a block (ArgumentError)

I've read several questions without being able to fix the problem. I am relatively new to Ruby, and just getting started with Rails. I'm probably a bit over my head.