Searchlogic OR condition

I'm using the gem 'searchlogic' for a search and I can not
use OR condition in eg the following:

Person.search person = (: birth_date_lte => Time.now.srftime ('% Y-% m-%
d)
: birth_date_null => null)

The SQL generated behind is:

SELECT person .* FROM person WHERE (birth_date IS NULL) AND (birth_date
<= '2011-08-16 ')

but I need to be an OR! The solution, i think, would be the following
(but not
works ..)

Person.search person = (: birth_date_null_or_lte =>
Time.now.srftime ('% Y-% m-% d))

Any ideas? Thanks!