CountDistinct in report viewer

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reporting

Hi!

I have a case here:


In my table I want to display the count of my data with letter that starts with "B".
my sample expression is this:

=CountDistinct(IIF(Left(Fields!Letter.Value,1) = "B",Fields!letter.Value,0))

The problem is all of my values are counted. the result should be:
with letter B = 2
without letter B = 1
but in my case 3 counts are displayed for those with letter B.


please help.
thanks :)

[edit: codeblock styling added]