diff --git a/simple64-gui/netplay/joinroom.cpp b/simple64-gui/netplay/joinroom.cpp
index 3b5c34e..68b46f2 100644
--- a/simple64-gui/netplay/joinroom.cpp
+++ b/simple64-gui/netplay/joinroom.cpp
@@ -308,7 +308,9 @@ void JoinRoom::processTextMessage(QString message)
         }
         else
         {
-            msgBox.setText(json.value("message").toString());
+            QString msg = json.value("message").toString();
+            if(msg == "Bad authentication code") msg += "<br>Note: using the official netplay servers is not allowed on unofficial builds.<br>You can host your own server with `simple64-netplay-server`";
+            msgBox.setText(msg);
             msgBox.exec();
         }
     }