Case statement in SQL

"PaymentReleaseDate"=
Case ISNULL(Convert(Varchar(20),tgr.ap_print_date,101),'') when '' then
( Select milestone_release_date from t_grant_milestones where grant_id= tgr.grant_id and entry_id=tgm.entry_id)
else tgr.ap_print_date 
End AS  PaymentReleaseDate,