I am getting a program error "uniqueness of index is violated"

I am using a simple form with two text boxes, text1 control source is
variable mhlotno, numerical, format 666666

Lotno is a table, with one field,named lotno, numerical,width of 6 and with a candidate cdx index named lotno

here is my script:

under gotfocus procedure for text1
mhlotno=0

under lostfocus procedusre for text1
SELECT lotno
SET ORDER to lotno

SEEK mhlotno
IF FOUND()
MESSAGEBOX("This Lotno already exists",0,-1,"New Lot Entry")
thisform.text2.SETfocus

ELSE
MESSAGEBOX("Woo Hoo New Lot made",0,-1,"New Lot Entry")
APPEND BLANK
replace lotno WITH mhlotno
thisform.text2.SETfocus
ENDIF