Update query for udf copy udf value from purhase order to purchase request

hi every one,

i accidentally update a udf in purchase request wrongly. and it affect same value in all purchase request document. now i want to update purchase request udf with purchase order. i write the query but have some problem.plz help me

update prq1

set prq1.U_U_S=(select por1.U_U_S

                 from

                 por1 inner join

                 PRQ1 on POR1.BaseLine = prq1.LineNum AND por1.BaseEntry = prq1.DocEntry and por1.Basetype = prq1 .ObjType

                 and por1.ItemCode=PRQ1.ItemCode

                 )

  from PRQ1