build process failing

I am unable to call the build method for a has_many :through relation:

class League < ActiveRecord::Base
  has_many :league_admins
  has_many :league_users, :through => :league_admins, :source => :user
end

class User < ActiveRecord::Base
  has_many :league_admins
  has_many :leagues, :through => :league_admins
end

# Conotroller
def create
    @league = current_user.leagues.build(params[:league])
    if @league.save
       ...
    end
end

# Error
RuntimeError in Admin/leaguesController#create
[]
RAILS_ROOT: /Users/chris/Documents/Projects/Rails/myleague
Application Trace | Framework Trace | Full Trace
app/controllers/admin/leagues_controller.rb:11:in `create'

Has something changed in Rails 2.1?