When tunelp is invoked with the '-i' option, and the option value is not a number, the somewhat confusing error "sscanf error: Success" is reported. tunelp /dev/lp0 -i x sscanf error: Success tunelp-1.3-10
You are right, that's clearly a mistake, and I've forwarded it on to the util-linux maintainers list, as tunelp is now part of util-linux. In the meantime, I guess the workaround is "don't do that then..." :-)
Andries fixed it in the upstream sources, so we'll get the fix in the next util-linux release. Thanks!