Monthly average including no transactions

Dear friends,

When I have a transactions that in some case the month is skip means no activity in certain month, and I'm averaging my transaction using this DAX =

avg.monthly sales = AVERAGEX(
                    VALUES(Dates[Month]),
                    [net sales]
                    )

how to get the average factor to be the whole numbers of months ?

The Dates dimension is the usual date table which of course have all complete months, but because my transaction is for example have skip month like :

For monthly average shouldn't be it is 250 / 4 instead 250 /3 ?

Also because the months is deliberately chosen from slicer, so I did choose June 2021, provided it has value or not, I think I want all selected month to be included in the average. 

Thanks