diff --git a/packages/overture-schema-divisions-theme/src/overture/schema/divisions/types.py b/packages/overture-schema-divisions-theme/src/overture/schema/divisions/types.py
index 621cfc416..699bfc9ac 100644
--- a/packages/overture-schema-divisions-theme/src/overture/schema/divisions/types.py
+++ b/packages/overture-schema-divisions-theme/src/overture/schema/divisions/types.py
@@ -3,16 +3,18 @@
 from pydantic import Field
 
 from overture.schema.system.field_constraint import UniqueItemsConstraint
-from overture.schema.system.primitive import uint8
+from overture.schema.system.primitive import int32
 
 from .models import HierarchyItem
 
 AdminLevel = NewType(
     "AdminLevel",
     Annotated[
-        uint8,
+        int32,
         Field(
             description="Integer representing the division's position in its country's administrative hierarchy, where lower numbers correspond to higher level administrative units.",
+            ge=0,
+            le=16,
         ),
     ],
 )
diff --git a/packages/overture-schema-divisions-theme/tests/division_area_baseline_schema.json b/packages/overture-schema-divisions-theme/tests/division_area_baseline_schema.json
index f3e8c2abd..e86de89d9 100644
--- a/packages/overture-schema-divisions-theme/tests/division_area_baseline_schema.json
+++ b/packages/overture-schema-divisions-theme/tests/division_area_baseline_schema.json
@@ -457,7 +457,7 @@
       "properties": {
         "admin_level": {
           "description": "Integer representing the division's position in its country's administrative hierarchy, where lower numbers correspond to higher level administrative units.",
-          "maximum": 255,
+          "maximum": 16,
           "minimum": 0,
           "title": "Admin Level",
           "type": "integer"
diff --git a/packages/overture-schema-divisions-theme/tests/division_baseline_schema.json b/packages/overture-schema-divisions-theme/tests/division_baseline_schema.json
index a2ee027b5..3cf5a93dd 100644
--- a/packages/overture-schema-divisions-theme/tests/division_baseline_schema.json
+++ b/packages/overture-schema-divisions-theme/tests/division_baseline_schema.json
@@ -524,7 +524,7 @@
       "properties": {
         "admin_level": {
           "description": "Integer representing the division's position in its country's administrative hierarchy, where lower numbers correspond to higher level administrative units.",
-          "maximum": 255,
+          "maximum": 16,
           "minimum": 0,
           "title": "Admin Level",
           "type": "integer"
diff --git a/packages/overture-schema-divisions-theme/tests/division_boundary_baseline_schema.json b/packages/overture-schema-divisions-theme/tests/division_boundary_baseline_schema.json
index 336d2d484..6d18b17ce 100644
--- a/packages/overture-schema-divisions-theme/tests/division_boundary_baseline_schema.json
+++ b/packages/overture-schema-divisions-theme/tests/division_boundary_baseline_schema.json
@@ -370,7 +370,7 @@
       "properties": {
         "admin_level": {
           "description": "Integer representing the division's position in its country's administrative hierarchy, where lower numbers correspond to higher level administrative units.",
-          "maximum": 255,
+          "maximum": 16,
           "minimum": 0,
           "title": "Admin Level",
           "type": "integer"
diff --git a/packages/overture-schema-system/src/overture/schema/system/model_constraint/model_constraint.py b/packages/overture-schema-system/src/overture/schema/system/model_constraint/model_constraint.py
index 9b8e0c924..1d7d22a44 100644
--- a/packages/overture-schema-system/src/overture/schema/system/model_constraint/model_constraint.py
+++ b/packages/overture-schema-system/src/overture/schema/system/model_constraint/model_constraint.py
@@ -56,10 +56,6 @@ def __init__(self, name: str | None = None):
             )
         self.__name = name
 
-    def __validate_instance(self, model_instance: BaseModel) -> BaseModel:
-        self.validate_instance(model_instance)
-        return model_instance
-
     @property
     def name(self) -> str:
         """Returns the name of the constraint, e.g. "FooConstraint" or "@foo"."""
@@ -126,6 +122,15 @@ def decorate(self, model_class: type[BaseModel]) -> type[BaseModel]:
         metadata = Metadata.retrieve_from(model_class, Metadata()).copy()  # type: ignore[union-attr]
         model_constraints = (*ModelConstraint.get_model_constraints(model_class), self)
         metadata[_MODEL_CONSTRAINT_KEY] = model_constraints
+        # Capture the constraint in a closure rather than passing a bound method.
+        # Some Pydantic versions unwrap bound methods passed through __validators__
+        # and rebind `self` to the model instance, breaking the dispatch.
+        constraint = self
+
+        def _after_validator(model_instance: BaseModel) -> BaseModel:
+            constraint.validate_instance(model_instance)
+            return model_instance
+
         new_model_class = create_model(
             model_class.__name__,
             __config__=config,
@@ -135,7 +140,7 @@ def decorate(self, model_class: type[BaseModel]) -> type[BaseModel]:
             __validators__={
                 self.name: cast(
                     Callable[..., Any],
-                    model_validator(mode="after")(self.__validate_instance),
+                    model_validator(mode="after")(_after_validator),
                 )
             },
             __metadata__=metadata,