Null-values do not flow when using declarative fim-expression

Hi

I used the following FIM declarative expression in an outbound-sync-Rule to flow mobile-Number to AD:

IIF(Eq(EmployeeType,"contractor"),homeMobilePhone,mobilePhone)

I selected "Allow null value to flow to destination"

The if homeMobilePhone or mobilePhone is present, the Attribute flows as expected. But when homeMobilePhone or mobilePhone is not present FIM doesn't flow a null value to AD and therfore the attribute is not deleted.

Of course it's possible to handle this using an AD Management Agent extension - but I'd like handle such easy logic declarative.

best regards & thanks for any advice

Pirmin