sendmessage and mqtt service return function_clause

[EdmenGU]

mqtt server version:4.3.1
and this is my code:
public static void QueueStart()
{
Task.Factory.StartNew(async () =>
{
while (true)
{
try
{
if (queue.TryDequeue(out var data))
{
if (data != null)
{
await MqttSubHelper.PublishAsync(data.Topic , data.Message);
await Task.Delay(500);
}
}
}
catch (System.Exception e)
{
LogHelper.Error(e , "MqttQueneHelper_QueueStart");
}
finally
{
await Task.Delay(50);
}
}
});
}
when send then message ,The server has a chance to reply function_clause.I donnot know how i and do .and the same server ,java has no problem