Want the Average of sales for months up to date, with empty months included

Question

How do I create an MDX that gives me the average of , f.ex., Sales
Note that some months may not have sales, but the month should still be counted.
The result I want to get out is a listing of Years with average of Sales for each year. Where current year only calculates up to current month.
Lets say that todays month is 2014-05 and we have the following figures for this year.
Month Sales
2014-01 10
2014-02 20
2014-03 Null
2014-04 15
2014-05 10
The calculation should be Sum(Sales) divided by 5 months: 55/5 = 11
If I use:
Avg( Descendants([Date].[Calendar].[Month].CurrentMemeber
, [Date].[Calendar].[Month])
, [Measures].[Internet Sales]
It will exclude 2014-03, which is wrong!
If I use CoalesceEmpty() to fill in the Nulls:
Avg( Descendants([Date].[Calendar].[Month].CurrentMemeber
, [Date].[Calendar].[Month])
, CoalesceEmpty([Measures].[Internet Sales],0)
It will fill in 0 for all months 2014 (up to 2014-12), which is also wrong!
It feels like a really simple calculation, but I can't make it work.
Thanks

Answer 1

I finally had to fix it by checking the number of months to get it right.
This solved my problem and gave me the correct averages on year and month level.
Create Member CurrentCube.[Measures].[COMPLAINT COUNT AVG CALC] As Null
,Format_String = ";;;\N\/\A",VISIBLE = 1;
    // For Year level - Show average of year = Sum/NumOfMonth. If this year then up to this month else divide by 12
    SCOPE([COMP CREATION DATE].[YEAR].[YEAR], [Measures].[COMPLAINT COUNT AVG CALC]);
        This = IIF([COMP CREATION DATE].[IS THIS YEAR].&[Y],
                        DIVIDE(SUM([COMPLAINT Count]),  Format(now(), "MM")),
                        DIVIDE(SUM([COMPLAINT Count]),  12)
        Format_String(This)="# ### ##0";
        NON_EMPTY_BEHAVIOR(This) = { [Measures].[COMPLAINT Count] };
    END SCOPE;
    // For month level - Show average of month = Sum/NumOfDaysInMonth
    SCOPE( [COMP CREATION DATE].[Year-Month-Date].[YEAR MONTHNO], [Measures].[COMPLAINT COUNT AVG CALC]);
        This = IIF(not isempty([Measures].[COMPLAINT Count]),
                Avg(
                    Descendants(
                     [COMP CREATION DATE].[Year-Month-Date].CurrentMember,
                     [COMP CREATION DATE].[Year-Month-Date].[DATE ID]
                    CoalesceEmpty([Measures].[COMPLAINT Count],0)
                    ,Null
        Format_String(This)="# ### ##0";
        NON_EMPTY_BEHAVIOR(This) = { [Measures].[COMPLAINT Count] };
    END SCOPE;
mStenport