diff --git a/README.rst b/README.rst
index e36f0909..0551dd76 100644
--- a/README.rst
+++ b/README.rst
@@ -174,7 +174,7 @@ Folly Format      folly::format   2.23
 {fmt} is the fastest of the benchmarked methods, ~35% faster than ``printf``.
 
 The above results were generated by building ``tinyformat_test.cpp`` on macOS
-10.14.6 with ``clang++ -O3 -DSPEED_TEST -DHAVE_FORMAT``, and taking the best of
+10.14.6 with ``clang++ -O3 -DNDEBUG -DSPEED_TEST -DHAVE_FORMAT``, and taking the best of
 three runs. In the test, the format string ``"%0.10f:%04d:%+g:%s:%p:%c:%%\n"``
 or equivalent is filled 2,000,000 times with output sent to ``/dev/null``; for
 further details refer to the `source