Formula to subtract 2 values in each group

@Nonprofit Reports & Dashboards

Hi- We're a nonprofit affordable housing developer and we need to know what the net unit move-ins are from the last month to this month.  We've built a report that has 2 groupings: Property and Monthly KPIs.  We've filtered the report to only show the most recent 2 Monthly KPIs for each Property.  The net move-in for each property is equal to the 1st Monthly KPI "Total Vacant" field value minus the 2nd Monthly KPI "Total Vacant" field value.  We want the net move in to show in the summary for each Property property.  I've gotten really close with this formula: 

Monthly_KPI__c.BP_Total_Vacant__c:SUM - PREVGROUPVAL(Monthly_KPI__c.BP_Total_Vacant__c:SUM, FK_NAME) 

*where FK_NAME is the API Name of the Monthly KPI Group

But I'm getting the error Incorrect argument type for function 'PREVGROUPVAL()'. When I try to display the result in the Development row group.    Any idea how I can fix?  Thanks.   Screenshot of report and formula editor are below