diff --git a/quiche/quic/core/quic_connection_id_manager.cc b/quiche/quic/core/quic_connection_id_manager.cc
index 253d3bd..74d3a51 100644
--- a/quiche/quic/core/quic_connection_id_manager.cc
+++ b/quiche/quic/core/quic_connection_id_manager.cc
@@ -379,6 +379,8 @@
 std::vector<QuicConnectionId>
 QuicSelfIssuedConnectionIdManager::GetUnretiredConnectionIds() const {
   std::vector<QuicConnectionId> unretired_ids;
+  unretired_ids.reserve(to_be_retired_connection_ids_.size() +
+                        active_connection_ids_.size());
   for (const auto& cid_pair : to_be_retired_connection_ids_) {
     unretired_ids.push_back(cid_pair.first);
   }