Why IRQL got lowered

Hi Developers,
When I use Driver Verifier to monitor my driver(a redirector). I met a stop code 0xd5:
meaning IRQL is not consistent across a dispatch. I noticed the out IRQL was lower
than in IRQL. And the device argument for the stop code is NULL. How could this happen?
And what does this mean? Can anybody help?
Thanks a lot!!!
Lijun


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