Error about strange entry in Principal!

Hello!

in the log of my RT3.0.11 instance I can see a lot of

[crit]: Found a principal () that was neither a user nor a group
(/usr/share/request-tracker3/lib/RT/Principal_Overlay.pm:93)

In my understanding of the logical data model. All Users or
Groups have a corresponding Entry in Principals.

So in my understanding the query

select Principals.id,PrincipalType from Principals
left join Users on Principals.id=Users.id
left join Groups on Principals.id=Groups.id
where Users.id is NULL and Groups.id is NULL;

should give an empty result set. BUT I got
nearly 150000 group entries. I looked at several
of the ids and found them referenced in the table
CachedGroupMember???

Any suggestions?? I think it would be nice to hav a tool
which checks the data consistency of the RT database.

best regards!