diff --git a/libguile/bytevectors.c b/libguile/bytevectors.c
index 7cd753009..1d4129705 100644
--- a/libguile/bytevectors.c
+++ b/libguile/bytevectors.c
@@ -79,7 +79,7 @@
   _sign char *c_bv;						\
 								\
   SCM_VALIDATE_##validate (1, bv);                              \
-  c_index = scm_to_uint (index);				\
+  c_index = scm_to_size_t (index);				\
 								\
   c_len = SCM_BYTEVECTOR_LENGTH (bv);				\
   c_bv = (_sign char *) SCM_BYTEVECTOR_CONTENTS (bv);		\