Trying to upgrade to latest Mongoid.

When I try and access elements in one of my models, I get the following error:

> reload! Event.first
NoMethodError: undefined method `each' for nil:NilClass
from /Users/danny/.rvm/gems/ruby-2.0.0-p247/gems/mongoid-4.0.0.alpha1/lib/mongoid/relations/eager/base.rb:32:in `set_on_parent'
from /Users/danny/.rvm/gems/ruby-2.0.0-p247/gems/mongoid-4.0.0.alpha1/lib/mongoid/relations/eager/has_and_belongs_to_many.rb:22:in `block in preload'
from /Users/danny/.rvm/gems/ruby-2.0.0-p247/gems/mongoid-4.0.0.alpha1/lib/mongoid/relations/eager/has_and_belongs_to_many.rb:21:in `each'
from /Users/danny/.rvm/gems/ruby-2.0.0-p247/gems/mongoid-4.0.0.alpha1/lib/mongoid/relations/eager/has_and_belongs_to_many.rb:21:in `preload'
from /Users/danny/.rvm/gems/ruby-2.0.0-p247/gems/mongoid-4.0.0.alpha1/lib/mongoid/relations/eager/base.rb:17:in `run'
from /Users/danny/.rvm/gems/ruby-2.0.0-p247/gems/mongoid-4.0.0.alpha1/lib/mongoid/relations/eager.rb:42:in `block in preload'
from /Users/danny/.rvm/gems/ruby-2.0.0-p247/gems/mongoid-4.0.0.alpha1/lib/mongoid/relations/eager.rb:41:in `each'
from /Users/danny/.rvm/gems/ruby-2.0.0-p247/gems/mongoid-4.0.0.alpha1/lib/mongoid/relations/eager.rb:41:in `preload'
from /Users/danny/.rvm/gems/ruby-2.0.0-p247/gems/mongoid-4.0.0.alpha1/lib/mongoid/relations/eager.rb:33:in `eager_load'
from /Users/danny/.rvm/gems/ruby-2.0.0-p247/gems/mongoid-4.0.0.alpha1/lib/mongoid/relations/eager.rb:23:in `eager_load_one'
from /Users/danny/.rvm/gems/ruby-2.0.0-p247/gems/mongoid-4.0.0.alpha1/lib/mongoid/relations/eager.rb:17:in `block in with_eager_loading'
from /Users/danny/.rvm/gems/ruby-2.0.0-p247/gems/mongoid-4.0.0.alpha1/lib/mongoid/contextual/mongo.rb:650:in `selecting'
from /Users/danny/.rvm/gems/ruby-2.0.0-p247/gems/mongoid-4.0.0.alpha1/lib/mongoid/relations/eager.rb:14:in `with_eager_loading'
from /Users/danny/.rvm/gems/ruby-2.0.0-p247/gems/mongoid-4.0.0.alpha1/lib/mongoid/contextual/mongo.rb:201:in `block (2 levels) in first'
from /Users/danny/.rvm/gems/ruby-2.0.0-p247/gems/mongoid-4.0.0.alpha1/lib/mongoid/contextual/mongo.rb:537:in `with_sorting'
from /Users/danny/.rvm/gems/ruby-2.0.0-p247/gems/mongoid-4.0.0.alpha1/lib/mongoid/contextual/mongo.rb:200:in `block in first'
from /Users/danny/.rvm/gems/ruby-2.0.0-p247/gems/mongoid-4.0.0.alpha1/lib/mongoid/contextual/mongo.rb:449:in `try_cache'
from /Users/danny/.rvm/gems/ruby-2.0.0-p247/gems/mongoid-4.0.0.alpha1/lib/mongoid/contextual/mongo.rb:199:in `first'
from /Users/danny/.rvm/gems/ruby-2.0.0-p247/gems/mongoid-4.0.0.alpha1/lib/mongoid/contextual.rb:19:in `first'
from /Users/danny/.rvm/gems/ruby-2.0.0-p247/gems/mongoid-4.0.0.alpha1/lib/mongoid/findable.rb:120:in `first'
from (irb):5
from /Users/danny/.rvm/gems/ruby-2.0.0-p247/gems/railties-4.0.2/lib/rails/commands/console.rb:90:in `start'
from /Users/danny/.rvm/gems/ruby-2.0.0-p247/gems/railties-4.0.2/lib/rails/commands/console.rb:9:in `start'
from /Users/danny/.rvm/gems/ruby-2.0.0-p247/gems/railties-4.0.2/lib/rails/commands.rb:62:in `<top (required)>'
from /Users/danny/Documents/workspace/ca/script/rails:6:in `require'
from /Users/danny/Documents/workspace/ca/script/rails:6:in `<top (required)>'
from -e:1:in `load'
from -e:1:in `<main>'

I don't get this error on any of my other models.