Expression Issue

Hi ALL ,

I am working on a matrix based report . The Data set contains the following column .

Year,Month ,Type,Customer,Amount 

The matrix report in dev looks like below 

Column 2 expression work fine that is : =MonthName(Fields!month.Value) &"-"& Fields!year.Value

Column 3 expression =  =IIF(Previous(Fields!month.Value) is nothing,"",MonthName(IIF(Previous(Fields!month.Value) is nothing,1,Previous(Fields!month.Value)))&" - "&MonthName(Fields!month.Value)&" "& Fields!year.Value)

i get the below error

the value expression for the textbox has a scope parameter that is not valid for an aggregate function . The string parameter must be set to a string constant that is equal to either the name of a containing group

Kindly advice .

Thanks & Regards

Priya