Recv Function Crash ( Noblocking Mode)

Good morning
I try to make a Multithread socket .I pass the AcceptHandler in a new thread and Succesfully Workthread Start .
The socket By Default is Blocking mode
In blocking Mode Create Threads and can Send data to the Last Socket Successfully
If go to Set The Socket in NoBlocking Mode

Código:

Dim NonBlock As Long 
 NonBlock = 1 
 If ioctlsocket (nRet, FIONBIO, NonBlock) = SOCKET_ERROR Then 
 WriteLog "Socket Cannot Set NoBlocking" 
 End If

My project Run Succesfully Create the First thread and The Second(ListenThread).When Client Connect start the ClientThread(WorkThread)and Crash .
My Workthread

Código:

Public Function Workthread (ByVal Param As Long) As Long 
 Const MAX_BUFFER_LENGTH As Long = 8192 'Normal = 8192' MAX = 65536 
 Dim Buffa (1 To MAX_BUFFER_LENGTH) As Byte 
 Dim dBuff As String 
 Dim ReadBuff As String 
 Dim sRec As Long 
 tID = GetCurrentThreadId 
 Add_DataClientStore ClientCount, nRet, tID''/ / Store Data in a public type (sockid, threadid) 
 WriteLog "New Client Arrive with Socket ID" & nRet & "On Thread ID:" & tID''/ / write data to listbox 
 Do''While True 

 ClientStore (ClientCount). Sock_ID = nRet 
 sRec = recv (ClientStore (ClientCount). Sock_ID, Buffa (1), MAX_BUFFER_LENGTH, 0 &) 
 ReadBuff = StrConv (Buffa, vbUnicode) 
 [b]dBuff = Left $ (ReadBuff, sRec) [/b]
 WriteLog "Data Received.:" & DBuff & "Socket ID ..." & ClientStore (ClientCount). Sock_ID 
 End If 
 Loop 
 End Function

The line to Crash is
dBuff = Left$(ReadBuff, sRec)
If Someone know anything
Thanks for the time

p.s Attach a ScreenShot with 505 Threads
2:The Main thread and the Listenthread
503 :Clients
cpu usage =100%

This is the Error
http://i54.tinypic.com/11qh94w.jpg
thanks for the time
Good night

Public Function Workthread (ByVal Param As Long) As Long
ThreadInit.InitThread
Dim DBuff As String
Dim ReadBuff As String
Dim sRec As Long
tID = GetCurrentThreadId
Add_DataClientStore ClientCount, nRet, tID
WriteLog "New Client Arrive with Socket ID" & nRet & "On Thread ID:" & tID
Do''While True
DBuff = Space (4096)
ClientStore (ClientCount). Sock_ID = nRet
sRec = recv (ClientStore (ClientCount). Sock_ID, ByVal DBuff, Len (DBuff), 0)
If sRec <> 0 And sRec <> SOCKET_ERROR Then
ReadBuff = ReadBuff & Left (DBuff, sRec)
WriteLog "Data Received.:" & ReadBuff & "Socket ID ..." & ClientStore (ClientCount). Sock_ID
End If
DoEvents
Loop
End Function