Formula to subtract one column total from another column not working if one is zero

Content

I want to create a formula that calculates chats offered minus chats abandoned in <10 seconds.  The following formula works if the chats abandoned in less than 10 seconds calculation is greater than zero.  If there were no chats abandoned in less than 10 seconds, the formula produces a zero for chats offered.  Any assistance would be greatly appreciated.

count(chats.requested )- (sum(if((chats.termination_event IN (3,14)) & (date_diff(chats.completed,chats.requested)<10), 1)))

 

 

Code Snippet

             count(chats.requested )- (sum(if((chats.termination_event IN (3,14)) & (date_diff(chats.completed,chats.requested)<10), 1)))