diff --git a/foreign/c/primitives/racket.scm b/foreign/c/primitives/racket.scm
index 525e8c9..a6fd08e 100644
--- a/foreign/c/primitives/racket.scm
+++ b/foreign/c/primitives/racket.scm
@@ -59,7 +59,7 @@
 
 (define c-bytevector-u8-set!
   (lambda (c-bytevector k byte)
-    (ptr-set! c-bytevector _uint8 'abs k byte)))
+    (ptr-set! c-bytevector _int8 'abs k byte)))
 
 (define c-bytevector-u8-ref
   (lambda (c-bytevector k)