diff --git a/aiopenapi3/openapi.py b/aiopenapi3/openapi.py
index 734294a5..7e5a998f 100644
--- a/aiopenapi3/openapi.py
+++ b/aiopenapi3/openapi.py
@@ -396,6 +396,7 @@ def _init_operationindex(self, use_operation_tags: bool):
         self._operationindex = OperationIndex(self, use_operation_tags)
 
     @staticmethod
+
     def _get_combined_attributes(schema):
         """Combine attributes from the schema."""
         return (
diff --git a/aiopenapi3/v20/general.py b/aiopenapi3/v20/general.py
index 8f93bda7..ad07831a 100644
--- a/aiopenapi3/v20/general.py
+++ b/aiopenapi3/v20/general.py
@@ -1,5 +1,5 @@
 import typing
-from typing import Optional, Any
+from typing import Optional, Any, Union
 
 from pydantic import Field, ConfigDict, PrivateAttr
 
@@ -7,7 +7,8 @@
 from ..base import ObjectExtended, ObjectBase, ReferenceBase
 
 if typing.TYPE_CHECKING:
-    from .._types import SchemaType
+    from .schemas import Schema
+    from .parameter import Parameter
 
 
 class ExternalDocumentation(ObjectExtended):
@@ -31,7 +32,7 @@ class Reference(ObjectBase, ReferenceBase):
 
     ref: str = Field(alias="$ref")
 
-    _target: "SchemaType" = PrivateAttr()
+    _target: Union["Schema", "Parameter", "Reference"] = PrivateAttr(default=None)
 
     model_config = ConfigDict(extra="ignore")
 
diff --git a/aiopenapi3/v30/general.py b/aiopenapi3/v30/general.py
index 808e26f4..d4dc171f 100644
--- a/aiopenapi3/v30/general.py
+++ b/aiopenapi3/v30/general.py
@@ -33,7 +33,7 @@ class Reference(ObjectBase, ReferenceBase):
 
     ref: str = Field(alias="$ref")
 
-    _target: Union["Schema", "Parameter", "Reference"] = PrivateAttr()
+    _target: Union["Schema", "Parameter", "Reference"] = PrivateAttr(default=None)
 
     model_config = ConfigDict(
         extra="ignore",  # """This object cannot be extended with additional properties and any properties added SHALL be ignored."""
diff --git a/aiopenapi3/v31/general.py b/aiopenapi3/v31/general.py
index b612888b..7a681e06 100644
--- a/aiopenapi3/v31/general.py
+++ b/aiopenapi3/v31/general.py
@@ -34,7 +34,7 @@ class Reference(ObjectBase, ReferenceBase):
     summary: Optional[str] = Field(default=None)
     description: Optional[str] = Field(default=None)
 
-    _target: Union["Schema", "Parameter", "Reference", "PathItem"] = PrivateAttr()
+    _target: Union["Schema", "Parameter", "Reference", "PathItem"] = PrivateAttr(default=None)
 
     model_config = ConfigDict(
         # """This object cannot be extended with additional properties and any properties added SHALL be ignored."""
diff --git a/aiopenapi3/version.py b/aiopenapi3/version.py
index 493f7415..4fbec93e 100644
--- a/aiopenapi3/version.py
+++ b/aiopenapi3/version.py
@@ -1 +1 @@
-__version__ = "0.3.0"
+__version__ = "0.4.0a3"
diff --git a/pyproject.toml b/pyproject.toml
index da229b4e..4ba6ca03 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -17,7 +17,7 @@ dependencies = [
 ]
 requires-python = ">=3.8"
 readme = "README.md"
-license = {text = "MIT"}
+license = {text = "BSD-3-Clause"}
 classifiers = [
     "Development Status :: 3 - Alpha",
     "Environment :: Web Environment",