NULL to 0

What modification does this query need that if either total_score or
questions_answered have a value of "NULL" that it will be changed to 0
("ZERO")   Ron

SELECT SUM(`my_Bible_trivia_knowledge_questions_answered`.`score`) AS
total_score, `my_Bible_trivia_knowledge_profile`.`questions_answered` FROM
`my_Bible_trivia_knowledge_questions_answered` INNER JOIN
`my_Bible_trivia_knowledge_profile` ON
`my_Bible_trivia_knowledge_questions_answered`.`my_Bible_trivia_knowledge_profile_reference`
= `my_Bible_trivia_knowledge_profile`.`reference` WHERE
`my_Bible_trivia_knowledge_profile`.`ip_address` = '$ip_address'