diff --git a/changelog.d/257.md b/changelog.d/257.md
new file mode 100644
index 000000000..b3f1e1a5d
--- /dev/null
+++ b/changelog.d/257.md
@@ -0,0 +1 @@
+- Fix `AttributeError` in `cps.marriage_tax_reforms` when reform was instantiated with `child_education_levels` — replace nonexistent `np.is_in` (typo) with `np.isin`.
diff --git a/policyengine_uk/reforms/cps/marriage_tax_reforms.py b/policyengine_uk/reforms/cps/marriage_tax_reforms.py
index accc83f80..21635024d 100644
--- a/policyengine_uk/reforms/cps/marriage_tax_reforms.py
+++ b/policyengine_uk/reforms/cps/marriage_tax_reforms.py
@@ -19,7 +19,7 @@ def formula(person, period):
                 child_meets_age_condition = person("age", period) <= max_child_age
                 return benunit.any(child_meets_age_condition)
             if child_education_levels is not None:
-                child_meets_education_condition = np.is_in(
+                child_meets_education_condition = np.isin(
                     person("education_level", period).decode_to_str(),
                     child_education_levels,
                 )
@@ -110,7 +110,7 @@ def formula(person, period):
                 child_meets_age_condition = person("age", period) <= max_child_age
                 return benunit.any(child_meets_age_condition)
             if child_education_levels is not None:
-                child_meets_education_condition = np.is_in(
+                child_meets_education_condition = np.isin(
                     person("education_level", period).decode_to_str(),
                     child_education_levels,
                 )