IF THEN ELSE problem

[krazykasper]

I have a formula in my Crystal Report as follows.
It works fine except it doesn't identify records when Sum ({@ExcessUsageAsOfDecember31},{OptimizeIt.LeaseMasterContractId}) is NULL. I tried inserting IS NULL in several places but keep getting syntax errors. I appreciate it anyone can help me finish off this formula.

IF {OptimizeIt.ContractStartDate} in DateTime (2008, 04, 16, 00, 00, 00) to DateTime (2008, 12, 31, 00, 00, 00)
AND Sum ({@ExcessUsageAsOfDecember31}, {OptimizeIt.LeaseMasterContractId}) <=7600
AND Sum ({@ExcessUsageAsOfApril15}, {OptimizeIt.LeaseMasterContractId}) <=7600
THEN "1st Audit - Letter B"
ELSE "TBD"

Krazy (Bill) Kasper