Access 2010 Query using IIF and ISERROR

I am trying to accomplish the following in an Access 2010 query

Select  
    UNIT, DATE, Sum(IIF(ISERROR(A),NULL,A)) AS DLP_PERCENTAGE  
From  
    tableA;

where

A = (ACT-BASE)/BASE  

I get a generic O开发者_StackOverflow社区VERFLOW error. I am missing something obvious. I am trying to catch an error in the calculation and return NULL if an error exists or the result if no error. I have to do it in a query. Any ideas what I have overlooked?

SUM((ACT-BASE)/IIF(BASE=0, Null, BASE)) AS DLP_PERCENTAGE