diff --git a/src/packet_writer.cpp b/src/packet_writer.cpp
index 613f070..7e79a10 100644
--- a/src/packet_writer.cpp
+++ b/src/packet_writer.cpp
@@ -70,12 +70,12 @@ void PacketWriter::write(Packet& packet) {
 }
 
 void PacketWriter::write(PDU& pdu, const struct timeval& tv) {
-    PDU::serialization_type buffer = pdu.serialize();
     struct pcap_pkthdr header;
     memset(&header, 0, sizeof(header));
     header.ts = tv;
+    header.len = static_cast<bpf_u_int32>(pdu.advertised_size());
+    PDU::serialization_type buffer = pdu.serialize();
     header.caplen = static_cast<bpf_u_int32>(buffer.size());
-    header.len = static_cast<bpf_u_int32>(buffer.size());
     pcap_dump((u_char*)dumper_, &header, &buffer[0]);
 }