Searchlogic OR condition

I'm using the gem 'searchlogic' for a search and I can not use OR condition in eg the following: person = (: birth_date_lte => ('% 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 = (: birth_date_null_or_lte => ('% Y-% m-% d))

Any ideas? Thanks!