setf mem-ref doesn't understant struct typedefs

[3b]

(defcstruct a-
  (a :int))
(defctype a (:struct a-))

(with-foreign-object (b '(:struct a-))
  (setf (mem-ref b '(:struct a-)) '(a 1)))
;; OK

(with-foreign-object (b '(:struct a-))
  (setf (mem-ref b 'a) '(a 1)))
;; =>
There is no applicable method for the generic function
  #<STANDARD-GENERIC-FUNCTION
    CFFI:TRANSLATE-INTO-FOREIGN-MEMORY (10)>
when called with arguments
  ((A 1) #<CFFI::FOREIGN-TYPEDEF A>
   #.(SB-SYS:INT-SAP #X06C7FFF0))