Datamapper DataMapper

I have this very simple model example:

require "rubygems"
require "dm-core"

class Store
  include DataMapper::Resource
  
  property :id,             Serial
  property :key,            String, :length => 255
  property :val,            String, :length => 65535
  
  def self.write( k , v )
    s = first_or_create( :key => k )
    s.update( :val => v )
  end
  
  def self.read( k )
    s = first( :key => k )
    s.val if s
  end
  
  def self.delete( k )
    s = first( :key => k )
    s.destroy! if s
  end
  
end


DataMapper::Logger.new(STDOUT)
DataMapper.setup(:default, "sqlite3::memory:")
DataMapper.auto_migrate!

session = {
  :expires => 1231233252,
  :session_key => "1234873r6yiejkh3l4k",
  :uid => 1231234532
}

Store.write( "sess" , session.to_s )
p Store.read( "sess" )

And it gives me these errors:

can't convert String into Array
/Users/slaskis/.gem/ruby/1.8/gems/dm-core-0.10.1/lib/dm-core/model/property.rb:181:in `zip'
/Users/slaskis/.gem/ruby/1.8/gems/dm-core-0.10.1/lib/dm-core/model/property.rb:181:in `key_conditions'
/Users/slaskis/.gem/ruby/1.8/gems/dm-core-0.10.1/lib/dm-core/resource.rb:737:in `query'
/Users/slaskis/.gem/ruby/1.8/gems/dm-core-0.10.1/lib/dm-core/resource.rb:609:in `collection'
/Users/slaskis/.gem/ruby/1.8/gems/dm-core-0.10.1/lib/dm-core/resource.rb:724:in `eager_load'
/Users/slaskis/.gem/ruby/1.8/gems/dm-core-0.10.1/lib/dm-core/resource.rb:710:in `lazy_load'
/Users/slaskis/.gem/ruby/1.8/gems/dm-core-0.10.1/lib/dm-core/property.rb:596:in `send'
/Users/slaskis/.gem/ruby/1.8/gems/dm-core-0.10.1/lib/dm-core/property.rb:596:in `lazy_load'
/Users/slaskis/.gem/ruby/1.8/gems/dm-core-0.10.1/lib/dm-core/property.rb:487:in `get'
/Users/slaskis/.gem/ruby/1.8/gems/dm-core-0.10.1/lib/dm-core/model/property.rb:200:in `val'

I don't see why?