diff --git a/jambo/schema_converter.py b/jambo/schema_converter.py
index 2e46828..00819b4 100644
--- a/jambo/schema_converter.py
+++ b/jambo/schema_converter.py
@@ -5,7 +5,7 @@ from jambo.types import JSONSchema, RefCacheDict
 from jsonschema.exceptions import SchemaError
 from jsonschema.validators import validator_for
 from pydantic import BaseModel
-from typing_extensions import Optional
+from typing_extensions import MutableMapping, Optional
 
 
 class SchemaConverter:
@@ -17,8 +17,10 @@ class SchemaConverter:
     fields and types. The generated model can be used for data validation and serialization.
     """
 
+    _namespace_registry: MutableMapping[str, RefCacheDict]
+
     def __init__(
-        self, namespace_registry: Optional[dict[str, RefCacheDict]] = None
+        self, namespace_registry: Optional[MutableMapping[str, RefCacheDict]] = None
     ) -> None:
         if namespace_registry is None:
             namespace_registry = dict()
@@ -64,7 +66,7 @@ class SchemaConverter:
             :return: The generated Pydantic model.
         """
         if ref_cache is None:
-            ref_cache = dict()
+            ref_cache: RefCacheDict = dict()
 
         try:
             validator = validator_for(schema)
diff --git a/pyproject.toml b/pyproject.toml
index 199edcd..2f1ca77 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -17,6 +17,7 @@ classifiers = [
     "Programming Language :: Python :: 3.11",
     "Programming Language :: Python :: 3.12",
     "Programming Language :: Python :: 3.13",
+    "Programming Language :: Python :: 3.14",
 ]
 license = "MIT"
 readme = "README.md"