RunningValue using a textbox that uses count function

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SSRS

I have a textbox within a group called Month that uses the following expression:

=Count(IIF(Fields!la_loan_app_status.Value = "OP", 1, Nothing))

Its purpose is to count all of the rows whose la_loan_app_status has a value of "OP'. As is it works fine. Now I need to present a running value of this from month to month. I created the following expression which does not work.

=RunningValue(Count(IIF(Fields!la_loan_app_status.Value = "OP", 1, Nothing)),Sum,Nothing)

Any help on what I am doing wrong would be appreciated. It does not produce an error but the values are totally incorrect.