You're right in both instances, but it doesn't solve my problem.

1 the formula includes the formula cell.
I made a mistake in copy-pasting to the newsgroup... has nothing to do with
my problem

2 second arguemnet is optional
try =IRR(A1:A9)
This is not an option for me, because I need to know the IRR for a
percentage of 0,25 (and several other percentages for that matter), so I
can't leave out the second argument.

My question still stands: Why does it give this error?