FltGetInstanceContext and Volume dismount!!

I am implementing a minifilter which uses a cancel safe queue.
In my pre callbacks, I call FltGetInstanceContext().
As such it is working fine… But whenever a Volume dismount request comes, the debugger (WinDbg) displays a Access Violation msg. ( code = 0xc0000005). When i click “Go Unhandled Exception”, it gives a bug check 0x8e with Parameter 1 as STATUS_ACCESS_VIOLATION.
Is it that i cannot call FltGetInstanceContext() in my precallback for IRP_MJ_FILE_SYSTEM_CONTROL with Volume Dismount FSCTL.

What may be the problem?