问连接到binance api的Websocket返回错误

EN

const WebSocket = require('ws');

const conn = new WebSocket("wss://testnet-dex.binance.org/api/ws");
conn.onopen = function(evt) {
    conn.send(JSON.stringify({ method: "subscribe", topic: "trades", symbols: ["ETH_BNB"] })); // I did try using as an symbols 'BNB_USDT.B-B7C', but got the same error
}
conn.onmessage = function(evt) {
    console.info('received data', evt.data);
};
conn.onerror = function(evt) {
    console.error('an error occurred', evt.data);
};

received data {"method":"subscribe","error":{"error":"Invalid symbol(s)"}}

received data {"method":"subscribe","error":{"error":"Symbols [BNB_USDT.B-B7C] are not valid for topic: trades, for user: 100.x.x.x:3333 \u003e 100.xx.x.x:59558"}}