[plt-scheme] semantics of letrec in Lazy Scheme

Hello,

In the following Lazy Scheme code, u is bound to #<undefiend>, 
whereas u1 and u2 are to promises producing 5. 
Why is u exceptional? 

#lang lazy

(define u (letrec ([x y] [y 5]) x))
(define u1 (letrec ([x (print y)] [y 5]) x))
(define u2 (letrec ([x (begin (print y) y)] [y 5]) x))


Best,
Keiko