Custom Record Field a!customFieldSum() - Not Handling Null Values

HI all 

I have a record for sum insured values let's say he below:

id: Primary Key (QuoteID)

riskType1SumInsured: Aggregate field of all RiskType1 sum insureds

riskType2SumInsured: Aggregate field of all RiskType2 sum insureds

riskType3SumInsured: Aggregate field of all RiskType3 sum insureds

totalSumInsured: a!customFieldSum(riskType1SumInsured, riskType2SumInsured,riskType3SumInsured ) 

When all the risk type 1,2 and 3 sum insured values have a value (Not Null) the totalSumInsured works correctly, however as soon as one or many of the risk type 1,2 and 3 sum insured values are null, the totalSumInsured simply returns a null.

I have tried wrapping the risk type 1,2 and 3 sum insured values in the a!customFieldSum(), with a!defaultValue() as well is a!isnullOrEmpty(), but no change in behaviour.

Is there something i am missing?