#Error when using Sum() function in paginated report

When ever I am trying to sum a column, instead of getting a number if get #error. The column is filled with currency values. There are some cells that are blank so that could be the issue. 

Here are some things I have tried: 

=SUM(IIf(IsNothing(Fields!External_20_Review_Fee.Value),0,Fields!External_20_Review_Fee.Value))

=SUM(Fields!External_20_Review_Fee.Value)

=SUM(IIf(INT(Fields!External_20_Review_Fee.Value),0,INT(Fields!External_20_Review_Fee.Value))