'sed -i' fails if the file has sufficient perms but not its directory

I tried this, which won't work :

tinderbox@tor-relay ~ $ sed -i -e 's/j1/j2/g' perl-5.22/etc/portage/make.conf
sed: couldn't open temporary file perl-5.22/etc/portage/sedw9SFZf: Permission denied

whereas I'm allowed to edit that file as the user tinderbox

tinderbox@tor-relay ~ $ vi perl-5.22/etc/portage/make.conf

/me wonders if sed relies on having a temp copy of that file in the same dir or not.


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Toralf, pgp key: 872AE508 0076E94E