Rails: named_scope, lambda and blocks

I thought the following two were equivalent:

named_scope :admin, lambda { |company_id| {:conditions => ['company_id = ?', company_id]} }

named_scope :admin, lambda do |company_id| 
  {:conditions => ['company_id = ?', company_id]}
end

but Ruby is complaining:

ArgumentError: tried to create Proc object without a block

Any ideas?