diff --git a/quiche/quic/qbone/bonnet/tun_device_packet_exchanger.cc b/quiche/quic/qbone/bonnet/tun_device_packet_exchanger.cc
index d15c38c..7575359 100644
--- a/quiche/quic/qbone/bonnet/tun_device_packet_exchanger.cc
+++ b/quiche/quic/qbone/bonnet/tun_device_packet_exchanger.cc
@@ -185,6 +185,10 @@
   }
 
   if (is_neighbor_solicit) {
+    // We need the local interface MAC address to respond.
+    if (!eth_hdr_initialized_) {
+      InitializeEthHdr();
+    }
     // If we've received a neighbor solicitation, craft an advertisement to
     // respond with and write it back to the local interface.
     auto* icmp6_payload = packet.data() + kIcmp6PrefixLen;