diff --git a/changelog.d/fixed/fix-climate-action-category.md b/changelog.d/fixed/fix-climate-action-category.md
new file mode 100644
index 000000000..2790eebf0
--- /dev/null
+++ b/changelog.d/fixed/fix-climate-action-category.md
@@ -0,0 +1 @@
+Fixed climate_action_incentive_category enum crash that broke household_net_income and marginal_tax_rate for all household types.
diff --git a/policyengine_canada/tests/gov/cra/tax/income/credits/climate_action/climate_action_incentive_category.yaml b/policyengine_canada/tests/gov/cra/tax/income/credits/climate_action/climate_action_incentive_category.yaml
new file mode 100644
index 000000000..9e9a88029
--- /dev/null
+++ b/policyengine_canada/tests/gov/cra/tax/income/credits/climate_action/climate_action_incentive_category.yaml
@@ -0,0 +1,74 @@
+- name: Two-parent household with child — category defaults without crash
+  period: 2026
+  input:
+    people:
+      parent1:
+        age: 35
+        is_head: true
+        is_spouse: false
+        employment_income: 50_000
+      parent2:
+        age: 33
+        is_head: false
+        is_spouse: true
+      child:
+        age: 5
+        is_head: false
+        is_spouse: false
+    households:
+      household:
+        members: [parent1, parent2, child]
+        province_code: ONT
+        is_married: true
+  output:
+    climate_action_incentive_category:
+      - HEAD
+      - SPOUSE
+      - OTHER_CHILD
+
+- name: Single parent household — eldest child gets special category
+  period: 2026
+  input:
+    people:
+      parent:
+        age: 35
+        is_head: true
+        is_spouse: false
+        employment_income: 45_000
+      child1:
+        age: 10
+        is_head: false
+        is_spouse: false
+        is_dependant: true
+      child2:
+        age: 7
+        is_head: false
+        is_spouse: false
+        is_dependant: true
+    households:
+      household:
+        members: [parent, child1, child2]
+        province_code: ONT
+        is_married: false
+  output:
+    climate_action_incentive_category:
+      - HEAD
+      - ELDEST_CHILD_IN_SINGLE_PARENT_HOUSEHOLD
+      - OTHER_CHILD
+
+- name: Single person — head only
+  period: 2026
+  input:
+    people:
+      person:
+        age: 40
+        is_head: true
+        is_spouse: false
+        employment_income: 90_000
+    households:
+      household:
+        members: [person]
+        province_code: ONT
+  output:
+    climate_action_incentive_category:
+      - HEAD
diff --git a/policyengine_canada/tests/gov/cra/tax/income/credits/climate_action/integration_two_parent.yaml b/policyengine_canada/tests/gov/cra/tax/income/credits/climate_action/integration_two_parent.yaml
new file mode 100644
index 000000000..bc5efc3f6
--- /dev/null
+++ b/policyengine_canada/tests/gov/cra/tax/income/credits/climate_action/integration_two_parent.yaml
@@ -0,0 +1,53 @@
+- name: Two-parent household with child — household_net_income does not crash
+  period: 2026
+  absolute_error_margin: 1
+  input:
+    people:
+      parent1:
+        age: 35
+        is_head: true
+        is_spouse: false
+        employment_income: 50_000
+      parent2:
+        age: 33
+        is_head: false
+        is_spouse: true
+        employment_income: 30_000
+      child:
+        age: 5
+        is_head: false
+        is_spouse: false
+    households:
+      household:
+        members: [parent1, parent2, child]
+        province_code: ONT
+        is_married: true
+  output:
+    climate_action_incentive_category:
+      - HEAD
+      - SPOUSE
+      - OTHER_CHILD
+
+- name: Married couple no children — climate action does not crash
+  period: 2026
+  absolute_error_margin: 1
+  input:
+    people:
+      head:
+        age: 40
+        is_head: true
+        is_spouse: false
+        employment_income: 90_000
+      spouse:
+        age: 38
+        is_head: false
+        is_spouse: true
+    households:
+      household:
+        members: [head, spouse]
+        province_code: ONT
+        is_married: true
+  output:
+    climate_action_incentive_category:
+      - HEAD
+      - SPOUSE
diff --git a/policyengine_canada/variables/gov/cra/tax/income/credits/climate_action/climate_action_incentive_category.py b/policyengine_canada/variables/gov/cra/tax/income/credits/climate_action/climate_action_incentive_category.py
index fae5243d2..dc590c854 100644
--- a/policyengine_canada/variables/gov/cra/tax/income/credits/climate_action/climate_action_incentive_category.py
+++ b/policyengine_canada/variables/gov/cra/tax/income/credits/climate_action/climate_action_incentive_category.py
@@ -41,4 +41,5 @@ def formula(person, period, parameters):
                 ClimateActionIncentiveCategory.ELDEST_CHILD_IN_SINGLE_PARENT_HOUSEHOLD,
                 ClimateActionIncentiveCategory.OTHER_CHILD,
             ],
+            default=ClimateActionIncentiveCategory.OTHER_CHILD,
         )