diff --git a/src/shrpx_downstream.cc b/src/shrpx_downstream.cc
index 65f3e970..eb284aa7 100644
--- a/src/shrpx_downstream.cc
+++ b/src/shrpx_downstream.cc
@@ -150,6 +150,9 @@ Downstream::Downstream(Upstream *upstream, MemchunkPool *mcpool,
 
   http2::init_hdidx(request_hdidx_);
   http2::init_hdidx(response_hdidx_);
+
+  request_headers_.reserve(16);
+  response_headers_.reserve(32);
 }
 
 Downstream::~Downstream() {