Fix warnings on 64 bit platforms.
diff --git a/test/rdbx_driver.c b/test/rdbx_driver.c
index 36639ea..5107ecf 100644
--- a/test/rdbx_driver.c
+++ b/test/rdbx_driver.c
@@ -120,7 +120,7 @@
 void
 print_rdbx(rdbx_t *rdbx) {
   printf("rdbx: {%llu, %s}\n",
-	 (rdbx->index), v128_bit_string(&rdbx->bitmask));
+	 (unsigned long long)(rdbx->index), v128_bit_string(&rdbx->bitmask));
 }