diff --git a/docs/HTTP3.md b/docs/HTTP3.md
index 851a0c4684..b09fb94e57 100644
--- a/docs/HTTP3.md
+++ b/docs/HTTP3.md
@@ -58,6 +58,7 @@ Build nghttp3
      % cd ..
      % git clone -b v1.1.0 https://github.com/ngtcp2/nghttp3
      % cd nghttp3
+     % git submodule update --init
      % autoreconf -fi
      % ./configure --prefix=<somewhere2> --enable-lib-only
      % make