How to avoid warnings looping over a hash table

["Mark A. Tapia" <markt@dgp.toronto.edu>]

Using MCL2.0f3, when I use the extended loop construct 
to iterate over a hash table, the compiler sometimes 
generates a message indicating that a variable has not
been used. 

I'd like to clean up the warning messages but don't
know how.

Thanks

(defvar a-hash-table (make-hash-table :test #'equal))
(setf (gethash '(1 2 3) a-hash-table) t)
(defun dump-hash (table)
  (when (hash-table-p table)
    (loop for key being the hash-key in table using (hash-value val)
               do (print-db key val))))
(dump-hash a-hash-table)
---> output
;Compiler warnings :
;   Unused lexical variable #:G15431, in DUMP-HASH.
KEY (1 2 3)
VAL T
NIL