my usb camera filter driver can not work on win 64bits

Hi All,

anybody expert on win 64bits pls help me, my filter driver work well on win
32 bits, but can not work on win 64bits, problem from Irp->MdlAddress always
null when i call from callback functions.
following is my code:

IoCopyCurrentIrpStackLocationToNext(Irp);
IoSetCompletionRoutine(Irp, (PIO_COMPLETION_ROUTINE)DeviceIoCompletion,
(PVOID)pdx, TRUE, TRUE, TRUE );
status = IoCallDriver(pdx->NextLowerDriver,Irp);

NTSTATUS DeviceIoCompletion(IN PDEVICE_OBJECT fido, IN PIRP Irp, IN
PDEVICE_EXTENSION pdx)
{
if(!Irp->MdlAddress)
DebugPrint(("Invalid Mdl Address"));
return STATUS_CONTINUE_COMPLETION;
}

With win 32 bits MdlAddress always != NULL, but with win 64 bits it always =
NULL, how different between 64 and 32 bits

Thanks very much.