Weird issue in Webi

A query is based on eFashion universe and returns two objects: measure [Promotion Cost USD] and dimension [Duration].

The variable [Cost] is defined as

=If [Duration]>0 Then [Promotion Cost USD]/[Duration]

The following formula in report’s cell returns #MULTIVALUE

=Sum([Cost])

If I just replace the variable with the expression, it will return correct value

=Sum(If [Duration]>0 Then [Promotion Cost USD]/[Duration])

The following formula also returns correct value

=Sum([Cost] ForEach([Duration]))

context issue