WebClient does not support concurrency

Test code is as follows

Imports System.Net
Imports System.Threading

Module Module1

    Sub Main()
        Dim c As New WebClient
        Dim t1 = c.DownloadStringTaskAsync("http://www.baidu.com")
        Dim t2 = c.DownloadStringTaskAsync("http://www.zhihu.com")
        Thread.Sleep(10)
        Console.WriteLine(t1. IsCompleted)
        Console.WriteLine(t1. IsCompleted)
    End Sub

End Module

Results are as follows

Unhandled exception: System.NotSupportedException:WebClient does not support concurrent I/O operations.
   In the System.Net.WebClient.ClearWebClientState ()
   At System.Net.WebClient.DownloadStringAsync (Uri address, Object userToken)
   At System.Net.WebClient.DownloadStringTaskAsync (Uri address)
   At System.Net.WebClient.DownloadStringTaskAsync (String address)
   ConsoleApplication1.Module1.Main () ConsoleApplication1Module1.vb: line number 9

Test environment: VS2013.