Set with Null values

[#1]

Hi,

I've a "Maintenances" table with 2 fields: FailureType and MaintenacesCount.
I wuold like to count all the maintenances where FailureType is null.
I'm using the expression below, which works correctly:

sum(MaintenancesCount) - sum({<FailureType={*}>}MaintenancesCount)

As you see, I get what I need subtracting 2 sets.
Is there a way to describe a set of null values? Something like this (which doesnt work)

sum({<FailureType={null}>}MaintenanceCount)

Regards,
Corrado

[#2]

You can replace NULL value by the string value - "NULLFailureType"

LOAD
 ....
 if(isNull(FailureType, 'NULLFailureType', FailureType)) as FailureType
 ....

After that you can use your expression:

sum({<FailureType={"NULLFailureType"}>}MaintenanceCount)