Comment 5 for bug 1222758

There should be a file /etc/apt/sources.list.d/apt-build.list . Open it with e.g. gedit:

gksudo gedit /etc/apt/sources.list.d/apt-build.list

Then change the line

deb file:/var/cache/apt-build/repository apt-build main

to

deb [arch=amd64] file:/var/cache/apt-build/repository apt-build main

Finally run apt-build again and check if it works.

(I doubt this will solve you issue though but will check if non-multiarch apt-build repository causes the build error.)

If it doesn't work, please show your /etc/apt/sources.list (attach it to this bug) or manually verify you have deb-src entries enabled and can do a `apt-get source PACKAGE`