AS3: negotiating FTP-ES connection - actionscript-3

Question

I'm trying to modify an ActionScript 3 FTP library to support explicit secure (FTP-ES) connections.
With FTP-ES, the client initially makes an insecure FTP connection, then explicitly asks the server to switch to a secure SSL/TLS connection.
Right now, I initially connect to the FTP server with a regular Socket, ask the server to switch to TLS, and then try connect with a SecureSocket after the server returns FTP code 234:
if (code.indexOf('220 ') == 0 || code.indexOf('220-') == 0) { //Send user name
if (_useTLS) {
trace("AUTH TLS");
cmdSocket.writeUTFBytes('AUTH TLS\r\n');
} else {
cmdSocket.writeUTFBytes('USER ' + _username + '\r\n');
}
cmdSocket.flush();
}
if (responseCode.indexOf('234 ') == 0) { //Auth OK, expecting SSL/TLS negotatiation
if (SecureSocket.isSupported) {
secureCommandSocket = new SecureSocket();
secureCommandSocket.addEventListener(ProgressEvent.SOCKET_DATA, onReceivedSCmd);
secureCommandSocket.addEventListener(Event.CONNECT, onConnectSCmd);
secureCommandSocket.addEventListener(IOErrorEvent.IO_ERROR, onSocketError);
secureCommandSocket.addEventListener(Event.CLOSE, onCloseSCmd);
secureCommandSocket.connect(_host, _port);
} else {
throw new Error("Secure socket is not supported on this platform");
}
}
However, the socket returns an IOError 2031 (socket error). Here's the output:
MESSAGE: CONNECTED TO FTP
MESSAGE: Received command: 220---------- Welcome to Pure-FTPd [privsep] [TLS] ----------
220-You are user number 1 of 50 allowed.
AUTH TLS
MESSAGE: Received command: 234 AUTH TLS OK.
MESSAGE: 2031:Error #2031: Socket Error. URL: xx.xx.xxx.xx
ERROR: Connection failed
With no more information about what's going wrong, I have no idea why the secure socket connection is failing. Is it because I'm trying to switch from a regular Socket to a SecureSocket mid-connection? If so, is there any other way to handle this in AS3?