diff --git a/lib/scheme/extras.scm b/lib/scheme/extras.scm
index 713913f0..d7c10843 100644
--- a/lib/scheme/extras.scm
+++ b/lib/scheme/extras.scm
@@ -118,4 +118,4 @@
 (define (bytevector-copy-partial! from start end to at)
   (do ((i start (+ i 1)))
       ((= i end))
-    (bytevector-u8-set! to (+ i at) (bytevector-u8-ref from i))))
+    (bytevector-u8-set! to (+ (- i start) at) (bytevector-u8-ref from i))))