emerge with world parameter error

[Buchu]

Since yesterday i have an error when i use emerge -u world, i try emerge -e system or emerge portage or emerge python... but nothing done something the error is only with world parameter :

$ emerge -u world -p

These are the packages that I would merge, in order.

Calculating world dependencies
Traceback (most recent call last):
File "/usr/bin/emerge", line 1203, in ?
if not mydepgraph.syscreate(getworldlist()):
File "/usr/bin/emerge", line 337, in getworldlist
if portage.db[portage.root]["vartree"].dep_match(x) and portage.db[portage.root]["porttree"].dep_match(x):
File "/usr/lib/python2.2/site-packages/portage.py", line 2347, in dep_match
if not self.hasnode(mypkgdep):
File "/usr/lib/python2.2/site-packages/portage.py", line 1912, in hasnode
myreskey=self.resolve_key(mykey)
File "/usr/lib/python2.2/site-packages/portage.py", line 1861, in resolve_key
self.load(mykey)
File "/usr/lib/python2.2/site-packages/portage.py", line 2380, in load
if not os.path.isdir(self.root+"/var/db/pkg/"+mycat):
File "/usr/lib/python2.2/posixpath.py", line 184, in isdir
st = os.stat(path)
TypeError: stat() argument 1 must be (encoded string without NULL bytes), not str