diff --git a/md5.asd b/md5.asd
index 8cc1d5a..4829f27 100755
--- a/md5.asd
+++ b/md5.asd
@@ -15,5 +15,9 @@
   :maintainer "Pierre R. Mai <pmai@pmsf.de>"
   :licence "Public Domain"
   :version "2.0.2"
-  #+sbcl :depends-on #+sbcl ("sb-rotate-byte")
+  :depends-on (#+sbcl "sb-rotate-byte"
+               #-(or :cmu :sbcl
+                     (and :lispworks (not :lispworks4))
+                     :ccl :allegro)
+               "flexi-streams")
   :components ((:file "md5")))
diff --git a/md5.lisp b/md5.lisp
index b1dae39..05ed5d9 100755
--- a/md5.lisp
+++ b/md5.lisp
@@ -711,7 +711,12 @@ determined by the underlying implementation."
     #-(or :cmu :sbcl (and :lispworks (not :lispworks4)) :ccl :allegro)
     (if (<= char-code-limit 256)
         (md5sum-sequence string :start start :end end)
-        (error "md5:md5sum-string is not supported for your implementation."))))
+        (md5sum-sequence
+         (flexi-streams:string-to-octets string
+                                         :external-format
+                                         (if (eq external-format :default)
+                                             :UTF-8
+                                             external-format))))))
 
 (eval-when (:compile-toplevel :load-toplevel :execute)
   (defconstant +buffer-size+ (* 128 1024)