diff --git a/api/features/multivariate/serializers.py b/api/features/multivariate/serializers.py
index abb09e0823ec..bf965cab4813 100644
--- a/api/features/multivariate/serializers.py
+++ b/api/features/multivariate/serializers.py
@@ -61,6 +61,14 @@ def get_control_value(self, obj: dict[str, typing.Any]) -> str | int | bool | No
 
 
 class MultivariateFeatureOptionSerializer(NestedMultivariateFeatureOptionSerializer):
+    # The model field has `default=100`, which only makes DRF mark this field
+    # `required=False` — it does not carry the default into `validated_data`.
+    # Declare it explicitly so omitting it from the request does not raise a
+    # `KeyError` in `validate()` below.
+    default_percentage_allocation = serializers.FloatField(
+        default=100, min_value=0, max_value=100
+    )
+
     class Meta(NestedMultivariateFeatureOptionSerializer.Meta):
         fields = NestedMultivariateFeatureOptionSerializer.Meta.fields + ("feature",)  # type: ignore[assignment]
         read_only_fields = ("uuid",)  # type: ignore[assignment]
diff --git a/api/tests/integration/features/multivariate/test_integration_multivariate.py b/api/tests/integration/features/multivariate/test_integration_multivariate.py
index 5067d8f8c697..cde3c424a4a4 100644
--- a/api/tests/integration/features/multivariate/test_integration_multivariate.py
+++ b/api/tests/integration/features/multivariate/test_integration_multivariate.py
@@ -44,6 +44,32 @@ def test_create_mv_option__valid_data__returns_created(  # type: ignore[no-untyp
     assert set(data.items()).issubset(set(response.json().items()))
 
 
+def test_create_mv_option__no_default_percentage_allocation__defaults_to_100(
+    admin_client_new: APIClient,
+    project: int,
+    feature: int,
+) -> None:
+    # Given - the request omits `default_percentage_allocation` entirely
+    url = reverse(
+        "api-v1:projects:feature-mv-options-list",
+        args=[project, feature],
+    )
+    data = {
+        "type": "unicode",
+        "feature": feature,
+        "string_value": "bigger",
+    }
+    # When
+    response = admin_client_new.post(
+        url,
+        data=json.dumps(data),
+        content_type="application/json",
+    )
+    # Then - it defaults to the model's default instead of a 500 KeyError
+    assert response.status_code == status.HTTP_201_CREATED
+    assert response.json()["default_percentage_allocation"] == 100
+
+
 def test_create_mv_option__with_key__returns_created_with_key(
     admin_client_new: APIClient,
     project: int,