diff --git a/src/network/protocols/server_lobby.cpp b/src/network/protocols/server_lobby.cpp
index 2db4e8ce1..d3b1730e7 100644
--- a/src/network/protocols/server_lobby.cpp
+++ b/src/network/protocols/server_lobby.cpp
@@ -463,6 +463,7 @@ void ServerLobby::signalRaceStartToClients()
  */
 void ServerLobby::startSelection(const Event *event)
 {
+    std::lock_guard<std::mutex> lock(m_connection_mutex);
     if (NetworkConfig::get()->isWAN())
     {
         assert(m_server_registered);
@@ -663,6 +664,7 @@ void ServerLobby::clientDisconnected(Event* event)
  */
 void ServerLobby::connectionRequested(Event* event)
 {
+    std::lock_guard<std::mutex> lock(m_connection_mutex);
     STKPeer* peer = event->getPeer();
     const NetworkString &data = event->data();
 
diff --git a/src/network/protocols/server_lobby.hpp b/src/network/protocols/server_lobby.hpp
index e2769ba73..dff62411b 100644
--- a/src/network/protocols/server_lobby.hpp
+++ b/src/network/protocols/server_lobby.hpp
@@ -6,6 +6,7 @@
 #include "utils/synchronised.hpp"
 
 #include <atomic>
+#include <mutex>
 #include <set>
 
 class ServerLobby : public LobbyProtocol
@@ -66,6 +67,10 @@ private:
     /** Timeout counter for showing the result screen. */
     float m_timeout;
 
+    /** Lock this mutex whenever a client is connect / disconnect or
+     *  starting race. */
+    std::mutex m_connection_mutex;
+
     // connection management
     void clientDisconnected(Event* event);
     void connectionRequested(Event* event);