problem with pprint dispatch

I'm trying to define my own printer function for the 'vector type, by  
means of set-pprint-dispatch.
In SBCL the following code works, but in OpenMCL it doesn't:

(set-pprint-dispatch
'vector
#'(lambda (stream object)
      (print-unreadable-object (object stream :type nil :identity nil)
        (format stream "OBJECT")
        (loop
           with *print-circle* = t
           with *print-level* = 1
           for slot across (subseq object 1)
           do (format stream " ~a" slot)))))

(format t "~a~%" (make-array 3 :initial-contents '(9 8 7)))

=> #<OBJECT 8 7>       (in SBCL - calls my custom vector printer)
=> #(9 8 7)                       (in OpenMCL - calls the default  
vector printer)

I use OpenMCL version 1.0 (beta: DarwinPPC64).