MO refundable EITC reform (`create_mo_refundable_eitc`) crashes when triggered

[PavelMakarchuk]

Summary

Triggering the bundled MO refundable-EITC contrib reform (gov.contrib.states.mo.child_poverty_impact_dashboard.eitc.in_effect = true) on policyengine-us==1.715.2 raises AttributeError: 'NoneType' object has no attribute 'entity' at calculation time. Two distinct bugs in policyengine_us/reforms/states/mo/eitc/mo_refundable_eitc_reform.py are at play.

Reproduce

from policyengine_us import Simulation
from policyengine_core.reforms import Reform

reform = Reform.from_dict({
    "gov.contrib.states.mo.child_poverty_impact_dashboard.eitc.in_effect": {"2026-01-01": True},
    "gov.states.mo.tax.income.credits.wftc.match": {"2026-01-01": 0.3},
}, country_id="us")

sim = Simulation(
    situation={
        "people": {"head": {"age": {2026: 35}, "employment_income": {2026: 40000}}},
        "tax_units": {"tu": {"members": ["head"]}},
        "households": {"hh": {"members": ["head"], "state_name": {2026: "MO"}}},
    },
    reform=reform,
)
sim.calculate("household_net_income", period=2026)

Bug 1 — add() receives a parameter path string

mo_refundable_credits.formula calls:

other_refundable = add(
    tax_unit,
    period,
    "gov.states.mo.tax.income.credits.refundable",
)

policyengine_core.commons.formulas.add expects variables: List[str] and iterates it:

for variable in variables:
    variable_entity = entity.entity.get_variable(variable).entity

When variables is a string, Python iterates character-by-character ("g", "o", "v", ".", …) and get_variable("g") returns None, producing AttributeError: 'NoneType' object has no attribute 'entity'.

Fix: resolve the parameter to its list value before handing it to add():

refundable_list = parameters(period).gov.states.mo.tax.income.credits.refundable
other_refundable = add(tax_unit, period, refundable_list)

Bug 2 — strict-check ValueError on PE-core 3.26.8 – 3.27.0

Before bug 1 even surfaces, on policyengine-core 3.26.8 through 3.27.0 you hit:

ValueError: Variable "mo_refundable_credits" mixes computation modes:
formula and adds/subtracts. Variables must use at most one of formula,
adds/subtracts, or uprating; plain input or constant variables should
use none.

mo_refundable_credits in the reform defines a formula but doesn't clear adds; the merge with the baseline (which has adds = "gov.states.mo.tax.income.credits.refundable") inherits the adds attribute. PE-core's strict computation-mode check rejects the combination.

PE-core 3.27.1 exempts baseline-inherited attributes from the check (changelog), so bumping core papers over bug 2. The proper fix is in this reform module: set adds = None (and subtracts = None) explicitly on any class that redeclares a formula via update_variable.

Expected behavior

The MO refundable-EITC reform should run without raising, converting the WFTC from nonrefundable to refundable.

Impact

Any caller setting the in_effect flag — including the child-poverty-impact-dashboard — crashes the simulation entirely (Bug 2) or silently produces household_net_income = 0 after the calling code swallows the AttributeError (Bug 1).

Environment

• policyengine-us==1.715.2
• policyengine-core>=3.26.8 (Bug 2); any version (Bug 1)
• Python 3.11

Workaround (downstream)

We've applied a runtime monkey-patch in our Modal endpoint that replaces create_mo_refundable_eitc with a corrected version, and pinned policyengine-core>=3.27.1. See our PR for the workaround we ship from the caller side.

[PavelMakarchuk]

Same bugs in other reforms

Did a static scan across policyengine_us/reforms/ for the same patterns.

Bug 1 (add() called with a parameter-path string) — one more

policyengine_us/reforms/states/ut/child_poverty_eitc/ut_fully_refundable_eitc_reform.py:49

class ut_non_refundable_credits(Variable):
    def formula(tax_unit, period, parameters):
        baseline_non_refundable = add(
            tax_unit,
            period,
            "gov.states.ut.tax.income.credits.non_refundable",
        )

Same fix as MO — resolve the parameter to its list value first:

non_refundable_list = parameters(period).gov.states.ut.tax.income.credits.non_refundable
baseline_non_refundable = add(tax_unit, period, non_refundable_list)

Reachable the same way: by setting gov.contrib.states.ut.child_poverty_impact_dashboard.eitc.in_effect = true. We've extended our downstream monkey-patch to cover this.

The pattern (add(entity, period, "<literal-path-string>")) does not appear anywhere else in policyengine_us/reforms/; the other reforms that touch state refundable-credits lists (sc_h3492, ny_s04487, al_hb527, abolish_payroll_tax, etc.) all resolve the parameter to a list before calling add().

Bug 2 (update_variable of a baseline that has adds, with the reform adding a formula)

Four additional reforms have this shape. They throw on policyengine-core 3.26.8 – 3.27.0 and pass on 3.27.1+ thanks to the inheritance-aware check. Still worth fixing in policyengine-us so the reforms don't depend on PE-core leniency:

• reforms/federal/abolish_payroll_tax.py → household_tax_before_refundable_credits
• reforms/states/al/hb527/al_hb527_overtime_deduction.py → al_agi
• reforms/states/ny/s04487/ny_s04487_newborn_credit.py → ny_refundable_credits
• reforms/treasury/repeal_dependent_exemptions.py → exemptions_count

For each, the reform's variable class should set adds = None (and subtracts = None if applicable) when it provides a formula, so the merge with the baseline doesn't carry forward adds/subtracts.