If statement in Reporting Services

I'm trying to add the attached code to a Reporting Services Dataset.  
When I debug the code in SQL Server the error I get is...
   The isnull function requires 2 arguments..
Would you please show me what I'm doing wrong?  

SELECT  IIF (IsNull(YEAR(tblClientInExit.ReIntakeDate)),YEAR(tblClients.OriginalStartDate),YEAR(tblClientInExit.ReIntakeDate)) AS PlaceYear,
DATEPART(quarter, tblClients.OriginalStartDate) AS PlacePeriod, tblClients.OriginalStartDate, 
                      tblAddresses.CountyLkUpID, tblClientInExit.ReIntakeDate
FROM         tblClients INNER JOIN
                      tblAddresses ON tblClients.ClientFileNo = tblAddresses.ClientFileNo INNER JOIN
                      tblClientInExit ON tblClients.ClientFileNo = tblClientInExit.ClientFileNo
ORDER BY PlacePeriod, tblClientInExit.ReIntakeDate DESC

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