SUSE zypper doesn't take piped inputs

Question

I have a problem with SUSE 12 zypper. I need to install an unsigned package with zypper in non-interactive mode from script. Unfortunately, zypper version is 1.13 and there is no option for flag "--allow-unsigned-rpm". So I've been trying to "pipe" output of the printf command in to zypper, but having no luck so far :( here is an example of what I am doing:

printf "y\ni\n" | sudo zypper install UNSIGNED_RPM_PATH_HERE

What am I doing wrong? This pipe works for every other script I tried to write myself.