rmdir: empty: Operation not permitted

[Petr Janda]

I cant delete a directory called empty.

porncatalog# rmdir empty
rmdir: empty: Operation not permitted
porncatalog# rm -rf empty
recursively remove empty? y
rm: empty: Operation not permitted
porncatalog# ls -la
total 6
drwxr-xr-x  3 root  wheel  512 Jul 23 18:44 .
drwxr-xr-x  3 root  wheel  512 Jul 23 18:44 ..
dr-xr-xr-x  2 root  wheel  512 Jul 22 18:34 empty
Im su root.

Whats the problem?