The weird thing is that this error does not occur when building on my
ordinary system (outside of the pbuilder chroot). Does pbuilder not
allow non-ASCII output or something?
Pbuilder is probably defaulting the locale to C, whereas your system/user locale is probably a UTF-8 locale (eg. en_US.UTF-8). You can probably resolve this issue with an `export LC_ALL=C.UTF-8` in debian/rules.