If one field null, populate another field with 0

Hello Splunkers, I have two fields that correlate. One field is hostname and another field is score. When I try to get an average of the score I get a incorrect value due to it calculating the score field even though the hostname is null and not representing anything. Is there a way to use if(isnull) or any other eval command so if hostname is null, it gives the other field the value of 0? Thanks, Cooper