plai signals unbound identifier for a field name

[robby]

In plai 1.10, the following code signals an error saying that 'value' is an unbound identifier, but it shouldn't I believe. (In 1.8, it signals no error.)

#lang planet plai/plai:1:10
(define-type DefrdSub?

[mtSub]
[aSub (value boolean?)])

(define (lookup ds the-name)

(type-case DefrdSub? ds

[mtSub () 1]
[aSub (a-name) 2]))