Getting "The underlying connection was closed: An unexpected error occurred on a receive." on FTPWebResponse.close() - vb.net

Question

I have a VB.Net function that will check if the FTP directory exists, otherwise create the directory.
The function works fine at first, but when the number of files in the FTP directory has reached a certain limit, response.close() in the finally block throws an exception : System.Net.WebException:The underlying connection was closed: An unexpected error occurred on a receive..
my function
Public Function funFTPCheckFolder(ByVal ftpPath As String, Optional ByVal bCreate As Boolean = False) As Boolean
Dim request As FtpWebRequest = Nothing
Dim response As FtpWebResponse = Nothing
Dim strReader As StreamReader = Nothing
Try
request = WebRequest.Create(ftpPath)
With request
.Credentials = New NetworkCredential(_username, _password)
.Method = WebRequestMethods.Ftp.ListDirectory
.KeepAlive = False
'.UsePassive = True
End With
response = CType(request.GetResponse, FtpWebResponse)
Catch ex As Exception
funFTPCheckFolder = False
If (bCreate = True) Then
request = WebRequest.Create(ftpPath)
With request
.Credentials = New NetworkCredential(_username, _password)
.Method = WebRequestMethods.Ftp.MakeDirectory
' .KeepAlive = False
.UsePassive = True
End With
response = CType(request.GetResponse, FtpWebResponse)
funFTPCheckFolder = True
End If
Exit Function
Finally
If response IsNot Nothing Then
response.Close()
response = Nothing
End If
If request IsNot Nothing Then
request.Abort()
End If
End Try
Return True
End Function
In this case, when the file amount in the FTP directory reached 481, an exception is thrown.
exception message
After moving one file to the subdirectory, it works fine again, so it seems to be a problem with the amount of file in the directory.
For now the only way I can come up with is wrap it with try/catch, but not sure will it cause any other error.
Try
If response IsNot Nothing Then
response.Close()
End If
Catch ex As Exception
End Try
I want to find out what exactly cause the problem, any thoughts would be appreciated!