diff --git a/md5.lisp b/md5.lisp
index 6bf04b6..5ac5a16 100644
--- a/md5.lisp
+++ b/md5.lisp
@@ -404,11 +404,15 @@ bounded by start and end, which must be numeric bounding-indices."
 	(declare (type (integer 0 63) amount))
 	(copy-to-buffer sequence start amount buffer buffer-index)
 	(setq start (the fixnum (+ start amount)))
-	(when (>= start end)
-	  (setf (md5-state-buffer-index state) (+ buffer-index amount))
-	  (return-from update-md5-state state)))
-      (fill-block-ub8 block buffer 0)
-      (update-md5-block regs block))
+        (let ((new-index (+ buffer-index amount)))
+          (when (= new-index 64)
+            (fill-block-ub8 block buffer 0)
+            (update-md5-block regs block)
+            (setq new-index 0))
+          (when (>= start end)
+            (setf (md5-state-buffer-index state) new-index)
+            (incf (md5-state-amount state) length)
+            (return-from update-md5-state state)))))
     ;; Handle main-part and new-rest
     (etypecase sequence
       ((simple-array (unsigned-byte 8) (*))