record_cache doesn't work with AR associations

[ninjudd]

This is being passed in the keys to find_by_field:
{:select=>nil, :include=>nil, :conditions=>nil, :readonly=>nil}

NoMethodError: undefined method <<' for nil:NilClass from vendor/record_cache/lib/record_cache/index.rb:280:inget_records'
from vendor/record_cache/lib/record_cache/index.rb:277:in each' from vendor/record_cache/lib/record_cache/index.rb:277:inget_records'
from vendor/record_cache/lib/record_cache/index.rb:270:in each_slice' from vendor/record_cache/lib/record_cache/index.rb:270:inget_records'
from vendor/memcache/lib/memcache.rb:264:in get_some' from vendor/record_cache/lib/record_cache/index.rb:262:inget_records'
from vendor/memcache/lib/memcache.rb:101:in in_namespace' from vendor/record_cache/lib/record_cache/index.rb:256:inget_records'
from vendor/record_cache/lib/record_cache/index.rb:105:in find_by_field' from vendor/record_cache/lib/record_cache.rb:218:infind_by_facebook_uid'
from vendor/rails/activerecord/lib/active_record/associations/belongs_to_association.rb:49:in send' from vendor/rails/activerecord/lib/active_record/associations/belongs_to_association.rb:49:infind_targ