Serial Port communication, getting error code 87 ERROR_INVALID_PARAMETER

Question

I was trying to access serial port data into excel 2002 (VBA) using:
CreateFile in overlapped mode to open com port
ReadFile to receive the data (with the overlapped structure passed on)

Declerations are:

Declare Function CreateFile Lib "kernel32" Alias "CreateFileA" _
    (ByVal lpFileName As String, ByVal dwDesiredAccess As Long, _
    ByVal dwShareMode As Long, lpSecurityAttributes As Any, _
    ByVal dwCreationDisposition As Long, ByVal dwFlagsAndAttributes As Long, _
    ByVal hTemplateFile As Long) As Long

Declare Function ReadFile Lib "kernel32" _
    (ByVal hFile As Long, ByVal lpBuffer As String, _
    ByVal nNumberOfBytesToRead As Long, lpNumberOfBytesRead As Long, _
     lpOverlapped As OVERLAPPED) As Long

Function being called as

    udtPorts(intPortID).lngHandle = CreateFile(strPort, GENERIC_READ Or _
        GENERIC_WRITE, 0, ByVal 0&, OPEN_EXISTING, FILE_FLAG_OVERLAPPED, 0)

and,

Dim lngStatus As Long
Dim lngRdSize As Long, lngBytesRead As Long
Dim lngRdStatus As Long, strRdBuffer As String * 1024

intPortID = 2

        lngRdStatus = ReadFile(udtPorts(intPortID).lngHandle, strRdBuffer, _
            lngRdSize, 0, udtCommOverlap)

                            lngStatus = GetLastError


After calling the function ReadFile, When the data is present in receive buffer, there is no error.

But if no data is available in receive buffer, i am getting the return value in 'lngRdStatus' as '0', and 'lngStatus' as 87.
The error discription given for this code is 'ERROR_INVALID_PARAMETER'.

I thought i should be getting 'ERROR_IO_PENDING' or 997.

Can anyone tell me what is going wrong. If required, i can paste the whole code..

The commTimeout values are:
     With udtCommTimeOuts
        .ReadIntervalTimeout = 5000
        .ReadTotalTimeoutMultiplier = 0
        .ReadTotalTimeoutConstant = 1000
        .WriteTotalTimeoutMultiplier = 0
        .WriteTotalTimeoutMultiplier = 1000
    End With