diff --git a/sha3.lisp b/sha3.lisp
index 22f3f6b..6edc0e7 100755
--- a/sha3.lisp
+++ b/sha3.lisp
@@ -110,7 +110,7 @@ and `end', which must be numeric bounding-indices."
              #.*optimize-declaration*)
     ;; Handle potential remaining bytes
     (unless (zerop buffer-index)
-      (let ((remainder (- (length buffer) buffer-index))
+      (let ((remainder (- rate-bytes buffer-index))
             (length (- end start)))
         (declare (type fixnum remainder length))
         (replace buffer vector :start1 buffer-index :start2 start :end2 end)