Empty Field in Cumulative Sum Calculation

I have a cumulative calculated member formula for story points resolved (story-punkte) on my burndown chart which works but there are empty fields on the days when there was no points burned. I need the empty fields to be filled with the same value as the field above i.e. first empty field would be 9 and the second empty field would be 23 etc. Please can you help? Here is the formula:

CASE
WHEN CurrentTuple(VisibleRowsSet()).Item(0).Name = ‘$total_aggregate’ THEN
[Measures].[Story-Punkte Resolved Status].Value
WHEN NOT IsEmpty([Measures].[Story-Punkte Resolved Status]) THEN
Sum(
Head(VisibleRowsSet(), Rank(CurrentTuple(VisibleRowsSet()), VisibleRowsSet())),
[Measures].[Story-Punkte Resolved Status]
)
END