=IFERROR(VLOOKUP(U4213,agt_ary!$A$2:$D$1500,2,FALS E),"Unknown ")

this is the function I am using now.
It will produce an unknow text is the data in the cell is not in the table
array.

however, for sorting porposes, if the U col is left blank it will still
produce the
unknow text,
how can I arrange this formula were if the U col data is not in the table
array
it will produce the "unkown" text, but it the col is left blank the cell
contianing the function will remail blank as well
please advise