panic in dqflush()

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

This afternoon, our 4.0/amd64 file server paniced with a null pointer  
dereference
in dqflush() during a quotaoff(8).

It looks like in
        if (dq->dq_ump->um_quotas[dq->dq_type] != vp)
dq->dq_ump was NULL (but not dq).

Does this ring a bell to anyone?
Sorry, I don't have a dump. Neither do I know of a way to reproduce it
(not that I wanted to on that machine, luckiliy it was sunday  
afternoon).