How do I fix a "Unknown configuration key `foreign-architecture' found in your `dpkg' configuration files." error?

Always when I install something, I get the following error multiple times:

Unknown configuration key 'foreign-architecture' found in your 'dpkg' configuration files. This warning will become a hard error at a later date, so please remove the offending configuration options and replace them with 'dpkg --add-architecture' invocations at the command line.

When I try dpkg --add-architecture I get:

Unknown configuration key `foreign-architecture' found in your `dpkg'
configuration files.  This warning will become a hard error at a later
date, so please remove the offending configuration options and replace
them with `dpkg --add-architecture' invocations at the command line.

dpkg: error: --add-architecture takes one argument

Type dpkg --help for help about installing and deinstalling packages [*];
Use `dselect' or `aptitude' for user-friendly package management;
Type dpkg -Dhelp for a list of dpkg debug flag values;
Type dpkg --force-help for a list of forcing options;
Type dpkg-deb --help for help about manipulating *.deb files;

Options marked [*] produce a lot of output - pipe it through `less' or `more' !

I've no problems yet, but since it says This warning will become a hard error at a later date I better do something about this. When I search 'foreign-architecture', I find an empty file, containing not a single byte. I somehow can't delete that file.

Please help, it's a kind of creapy...

I use Ubuntu version 12.10

Updated the questions:

cat /etc/dpkg/dpkg.cfg 
no-debsig 
log /var/log/dpkg.log

ls /etc/dpkg/dpkg.cfg.d/
multiarch 

Solution 1:

You say in the comments that you see:

$ ls /etc/dpkg/dpkg.cfg.d/
multiarch

Presumably that file, /etc/dpkg/dpkg.cfg.d/multiarch, contains a foreign-architecture key. So I suggest just deleting the file:

sudo rm /etc/dpkg/dpkg.cfg.d/multiarch

Solution 2:

My /etc/dpkg/dpkg.cfg.d/multiarch contained:

foreign-architecture i386

I deleted the file. I then issued:

dpkg --add-architecture foreign-architecture