diff --git a/quic/core/quic_connection.cc b/quic/core/quic_connection.cc
index 8203bce..3ba720f 100644
--- a/quic/core/quic_connection.cc
+++ b/quic/core/quic_connection.cc
@@ -384,8 +384,6 @@
   // TODO(ianswett): Supply the NetworkChangeVisitor as a constructor argument
   // and make it required non-null, because it's always used.
   sent_packet_manager_.SetNetworkChangeVisitor(this);
-  sent_packet_manager_.ReserveUnackedPacketsInitialCapacity(
-      GetUnackedMapInitialCapacity());
   if (GetQuicRestartFlag(quic_offload_pacing_to_usps2)) {
     sent_packet_manager_.SetPacingAlarmGranularity(QuicTime::Delta::Zero());
     release_time_into_future_ =
@@ -5514,5 +5512,10 @@
   OnSuccessfulMigration();
 }
 
+void QuicConnection::SetUnackedMapInitialCapacity() {
+  sent_packet_manager_.ReserveUnackedPacketsInitialCapacity(
+      GetUnackedMapInitialCapacity());
+}
+
 #undef ENDPOINT  // undef for jumbo builds
 }  // namespace quic
diff --git a/quic/core/quic_connection.h b/quic/core/quic_connection.h
index 823fa93..136eb90 100644
--- a/quic/core/quic_connection.h
+++ b/quic/core/quic_connection.h
@@ -946,6 +946,8 @@
   // connection ID lengths do not change.
   QuicPacketLength GetGuaranteedLargestMessagePayload() const;
 
+  void SetUnackedMapInitialCapacity();
+
   virtual int GetUnackedMapInitialCapacity() const {
     return kDefaultUnackedPacketsInitialCapacity;
   }
diff --git a/quic/core/quic_session.cc b/quic/core/quic_session.cc
index 2e1895e..058c775 100644
--- a/quic/core/quic_session.cc
+++ b/quic/core/quic_session.cc
@@ -134,6 +134,7 @@
   connection_->set_visitor(this);
   connection_->SetSessionNotifier(this);
   connection_->SetDataProducer(this);
+  connection_->SetUnackedMapInitialCapacity();
   connection_->SetFromConfig(config_);
   if (perspective_ == Perspective::IS_CLIENT &&
       config_.HasClientRequestedIndependentOption(kAFFE, perspective_) &&