Why isn't this case statement working in MS SQL server?

Gurus,

I have a table defined as follows:

create table WebUIUpgrade.AuditInvoiceLineCountsBeforeFinalSalesTaxLineWrites
    (InvoiceNumber_TIHLNUMINV numeric(10,0) not null,  
     TotalTargetLineCount int,
     SumTaxLineCount int,
     SumNonTaxLineCount int,
     TotalRestoreSourceLineCount int not null,
     InsertedDatetime datetime not null,
     countIsDifferent bit not null
    )

I want to (1) define the TotalTargetLineCount as the sum of the SumTaxLineCount + SumNonTaxLineCount, (2) compare the TotalRestoreSourceLineCount to the value of the defined TotalTargetLineCount, (3) set the countIsDifferent bit to 1 if TotalRestoreSourceLineCount and the TotalTartetLineCount don't match.

To do this, I used this SQL statement:

          select sum(SumNonTaxLineCount + SumTaxLineCount) as TotalTargetLineCount,
                 InvoiceNumber_TIHLNUMINV,
                 case countIsDifferent
                 when (TotalRestoreSourceLineCount - sum(SumNonTaxLineCount + SumTaxLineCount)) > 0                            then  '1'
                 else
                   '0'
               end as countIsDifferent
           from  WebUIUpgrade.AuditInvoiceLineCountsBeforeFinalSalesTaxLineWrites
            group by InvoiceNumber_TIHLNUMINV

The statement comes back with the error:

Msg 102, Level 15, State 1, Line 742
Incorrect syntax near '>'.

Any ideas on how to solve this?