video class lower filter

我写了一个video class的lower filter，现在能拦截到URB_FUNCTION_ISOCH_TRANSFER命令，我现在想在这里自己填充ｕｒｂ里面的数据直接返回给上层驱动，代码如下

ULONG NumberOfPackets = pUrb->UrbIsochronousTransfer.NumberOfPackets;
USBD_ISO_PACKET_DESCRIPTOR *pIsoPacket = pUrb->UrbIsochronousTransfer.IsoPacket;
PUCHAR pData = (PUCHAR)pUrb->UrbIsochronousTransfer.TransferBuffer;

for(i=0; i<(int)NumberOfPackets; i++)
{
ptr = pData + pIsoPacket->Offset;
pIsoPacket->Length = SendLen + 2;
pIsoPacket->Status = USBD_STATUS_SUCCESS;
....
TransCount += (SendLen+2);
pIsoPacket++;
｝

pUrb->UrbIsochronousTransfer.NumberOfPackets = i;
pUrb->UrbIsochronousTransfer.TransferBufferLength = TransCount;
pUrb->UrbHeader.Status = USBD_STATUS_SUCCESS;
Irp->IoStatus.Information = 0;
Irp->IoStatus.Status = STATUS_SUCCESS;
IoCompleteRequest(Irp, IO_NO_INCREMENT);

结果会导致蓝屏发生在usbvideo.sys驱动里
但如果我设置了完成例程，在完成例程里更改Ｕｒｂ中的数据内容，上层驱动就可以正确的收到更改后的内容了
我上面的代码哪里出错了，该怎么来完成这个功能