Can't set array-typed class slot value with accessor

I've got a class with a slot of type (simple-array (unsigned-byte 54) (* *)) (the 54 is not a typo, I have something that needs fifty-four bits (I'm aware it'll get upgraded to fixnum on 64-bit linux x86)). When I attempt to set this slot to the result of (make-array ...) of the proper type with its accessor, it fails:

? (defclass foo () ((x :accessor x :type (simple-array (unsigned-byte 54) (* *)))))
#<STANDARD-CLASS FOO>

? (setf (x (make-instance 'foo))  (make-array (list 3 4) :element-type '(unsigned-byte 54)))
> Error: The value #<ARRAY 3x4 type (UNSIGNED-BYTE 64), simple> can not be used to set the value of the slot X in #<FOO #x3020004C4B5D>, because it is not of type (SIMPLE-ARRAY (UNSIGNED-BYTE 54) (* *)).
> While executing: CCL::%MAYBE-STD-SETF-SLOT-VALUE-USING-CLASS, in process listener(1).
> Type :POP to abort, :R for a list of available restarts.
> Type :? for other options.
1 > :pop

? (lisp-implementation-version)
"Version 1.12-dev (v1.12-dev.5-23-gb25693fa) LinuxX8664"

Using slot-value instead of the accessor succeeds:

? (setf (slot-value (make-instance 'foo) 'x)  (make-array (list 3 4) :element-type '(unsigned-byte 54))) 
#2A((0 0 0 0) (0 0 0 0) (0 0 0 0))

Storing the array in a variable and using that with the accessor also succeeds:

? (defparameter *x* (make-array (list 3 4) :element-type '(unsigned-byte 54))) 
*X* 

? (setf (x (make-instance 'foo)) *x*) 
#2A((0 0 0 0) (0 0 0 0) (0 0 0 0))

Clozure / ccl

Can't set array-typed class slot value with accessor #291