You're doing nothing wrong; that's a base Zen Cart issue for the zc158 series.

See the following PRs for the changes: https://github.com/zencart/zencart/pull/5711/files superceded by https://github.com/zencart/zencart/p...c1485a45cc03f9

Note that the line numbers in the latter commit will be somewhat off from the zc158 release, due to module refactoring.