Go websocket error: close 1006 (abnormal closure): unexpected EOF

I'm trying to write a simple go websocket server with gorilla/websocket

http.HandleFunc("/ws", func(w http.ResponseWriter, r *http.Request) {
    if os.Getenv("env") == "development" {
        upgrader.CheckOrigin = func(r *http.Request) bool { return true }
    }

    conn, err := upgrader.Upgrade(w, r, nil)
    if err != nil {
        log.Printf("Websocket error: %s", err)
        return
    }

    defer conn.Close()

    // Register client
    clients[conn] = true

    for {
        message := message.Message{}
        _, msg, err := conn.ReadMessage()
        if err != nil {
            log.Printf("Websocket error: %s", err)
            return
        }

        res, _ = json.Marshal(context.Game)

        // Send to every client that is currently connected
        for client := range clients {
            err := client.WriteMessage(websocket.TextMessage, res)
            if err != nil {
                // Remove connection
                client.Close()
                delete(clients, client)
            }
        }
    }
})

the _, msg, err := conn.ReadMessage() line is throwing an error and closing the websocket, but I'm not sure why.

The error is close 1006 (abnormal closure): unexpected EOF. How can I prevent this?