Dax formula for cumulative - can't get it to show cumulatives in no change months

hi Folks
I got some help to create this formula - I need a formula to show cumulative month on month and this works beautifully for months where there is data.

if(sum(SLA_Source[sla_net_accounted])<>0,Calculate(sum(SLA_Source[sla_net_accounted]),
DATESBETWEEN(Acc_Dates[AccDate],FIRSTDATE(all(SLA_Source[gl_date])),
LASTDATE(SLA_Source[gl_date]))),blank())


However for months where there is no movement, it just shows a blank :-( whereas what I want it to show is the previous month's cumulative...
eg.
Jan actual
100
January cumulative
100
Feb actual
50
February cumulative
150
March actual
blank
March cumulative
Blank - whereas I want it to show 150 as well...

Thanks for your help in advance...

Anne




It works beautifu

Excel 2010 with free PowerPivot Add-In
Accumulate, missing month.
http://www.mediafire.com/download/ds47z8aqt87q07x/08_19_13.xlsx