diff --git a/util/guest-random.c b/util/guest-random.c
index 086115bd67..23643f86cc 100644
--- a/util/guest-random.c
+++ b/util/guest-random.c
@@ -38,7 +38,7 @@ static int glib_random_bytes(void *buf, size_t len)
     }
     if (i < len) {
         x = g_rand_int(rand);
-        __builtin_memcpy(buf + i, &x, i - len);
+        __builtin_memcpy(buf + i, &x, len - i);
     }
     return 0;
 }