Using CASE with other Views

Hi

I am trying to create a case based on 2 different views called dbo.MostRecentStockWIPRecord and dbo.MaxJVD working with null values. 

Here is the code I have created:

SELECT        mrswip.PalletID, mrswip.Description, mrswip.[Customer Code], mrswip.Reference, mrswip.[Item No],
                     mrswip.[Pallet Qty], mrswip.[Pallet No], mrswip.JobID, mrswip.BookedInDate
                             CASE  mrswip.[pallet qty]
                                   WHEN isnull THEN mjvd.[Pallet Quantity]
                                   ELSE mrswip.[pallet qty]
                             END AS DefinitivePalletQty
FROM            dbo.MostRecentStockWIPRecord AS mrswip LEFT OUTER JOIN
                         dbo.MaxJVD AS mjvd ON mrswip.PalletID = mjvd.[Pallet ID]

I keep getting the following error:

Does anyone know why?

Thanks

Aaron