diff --git a/src/network/network_server.cpp b/src/network/network_server.cpp
index 683774858e..f79f0a7ff3 100644
--- a/src/network/network_server.cpp
+++ b/src/network/network_server.cpp
@@ -138,13 +138,13 @@ struct PacketWriter : SaveFilter {
 
 	void Write(byte *buf, size_t size) override
 	{
+		std::lock_guard<std::mutex> lock(this->mutex);
+
 		/* We want to abort the saving when the socket is closed. */
 		if (this->cs == nullptr) SlError(STR_NETWORK_ERROR_LOSTCONNECTION);
 
 		if (this->current == nullptr) this->current = std::make_unique<Packet>(PACKET_SERVER_MAP_DATA, TCP_MTU);
 
-		std::lock_guard<std::mutex> lock(this->mutex);
-
 		byte *bufe = buf + size;
 		while (buf != bufe) {
 			size_t written = this->current->Send_bytes(buf, bufe);
@@ -161,11 +161,11 @@ struct PacketWriter : SaveFilter {
 
 	void Finish() override
 	{
+		std::lock_guard<std::mutex> lock(this->mutex);
+
 		/* We want to abort the saving when the socket is closed. */
 		if (this->cs == nullptr) SlError(STR_NETWORK_ERROR_LOSTCONNECTION);
 
-		std::lock_guard<std::mutex> lock(this->mutex);
-
 		/* Make sure the last packet is flushed. */
 		if (this->current != nullptr) this->packets.push_back(std::move(this->current));