[plt-scheme] [plai] bug with mutator?

Hi,

#lang plai
(define-type A (a (x number?)))

> (set-a-x! (a 10) 1)
. . procedure set-a-x!: expects 1 argument, given 2: (a 10) 1

how can the mutator expect only one argument?

even stranger:
> (set-a-x! (a 10))
. . #<procedure>: expects 2 arguments, given 1: (a 10)

is that a bug?

Thanks,

-- Éric