RuntimeWarning: invalid value encountered in divide — QBI / labor-supply variables (84 occurrences in policyengine.py test suite) #8216

Running pytest tests --no-cov on PolicyEngine/policyengine.py@joss-paper produces 429 passes and 84 RuntimeWarning: invalid value encountered in divide warnings. All originate from policyengine_us, concentrated in QBI (qualified business income) and labor-supply variables.

These are not test failures, but they create significant noise in the test output and likely indicate division-by-zero in formulas that should be guarded with np.where / np.divide(where=...).

Related but more specific than:

• Remove divide-by-zero causing warning in test suite #745 (general divide-by-zero in test suite)
• Runtime warning: Invalid value in rank calculation #4891 (rank-calc warning)
• Consider migrating from mask approach to np.divide for division by zero handling #6356 (mask → np.divide refactor proposal)

Reproduction:

git clone https://github.com/PolicyEngine/policyengine.py
cd policyengine.py && git checkout joss-paper
pip install -e .
pytest tests --no-cov -W error::RuntimeWarning  # to surface them

Reported in the discussion on PolicyEngine/policyengine.py#264.