Exent(foo1) not welcome here

My wierd bug for today

/* class foo */
CLASS foo:
     DEF PUBLIC PROPERTY bar AS CHAR NO-UNDO GET . SET .
END CLASS.
/* program a */
DEF VAR foo1 AS CLASS foo EXTENT NO-UNDO.
DEF VAR demo1 AS INT NO-UNDO.
EXTENT(foo1) = 5.
demo1 = EXTENT(foo1).
foo1[EXTENT(foo1)] = NEW foo().
foo1[EXTENT(foo1)]:bar = "you are not welcome here". /* this does not compile */
foo1[demo1]:bar = "come on in". /* this does */

the actual error for the line that does not compile is

Invalid unsubscripted reference to array element. (3531)
You must subscript array elements in WHERE clauses unless the reference is in a CAN-FIND in a VALIDATION expression of an array variable.