Use Case Statement in Join

HET zou moeten zijn,

ON 
ts.ACCOUNTID =  CASE
                    WHEN ts.ACCOUNTTYPE = '1' THEN SI.TENANCYID
                    WHEN ts.ACCOUNTTYPE = '2' THEN SI.EMPLOYEEID
                    WHEN ts.ACCOUNTTYPE = '3' THEN SI.SUPPLIERID
                    WHEN ts.ACCOUNTTYPE = '4' THEN SI.SALESCUSTOMERID
                END