Sum with empty(null) measures

When I was working on a report I ran into a problem when I tried to sum a measure that contained an empty cell.  I expected the empty cells to be treated as zero but that wasn’t the case.  The result returned was null.  Here’s the sample data.

District	Attendance Rate
A	(null)
A	0.0%
A	94.5%

My first solution was just to apply a filter to remove the empty cell from the set.

With Member Total AS   
SUM(EXISTING(FILTER([Attending School].[District], NOT ISEMPTY(Measures.[Attendance Rate])),Measures.[Attendance Rate])

The result was still null. I tried a variety of things to remove the empty cells from the calculated measure or transform them into zero but with no success.  The result was always the same, null for the sum instead of 94.5.

Consequently I moved away from manipulating the calculated member itself and came up with a solution in a subcube.  I was able to remove those cells without an attendance rate from the cube before summing.

WITH MEMBER Total AS 
SUM(Existing([Attending School].[District]),Measures.[Attendance Rate]))

SELECT Measures.Total ON COLUMNS,
[Attending School].[District] ON ROWS
FROM (SELECT  [Enrolled From Date].[Academic Calendar].[Academic Year] ON COLUMNS,
Filter([Attending School].[District] NOT ISEMPTY([Measures].[Attendance Rate])) ON ROWS
FROM [EdDW])
WHERE [Enrolled From Date].[Academic Calendar].[Academic Year].&[20082009]