diff --git a/scim2_models/rfc7643/resource.py b/scim2_models/rfc7643/resource.py
index ed89b4f..b8a68c8 100644
--- a/scim2_models/rfc7643/resource.py
+++ b/scim2_models/rfc7643/resource.py
@@ -4,7 +4,6 @@
 from typing import Generic
 from typing import Optional
 from typing import TypeVar
-from typing import Union
 from typing import get_args
 from typing import get_origin
 
@@ -24,6 +23,7 @@
 from ..base import Uniqueness
 from ..base import URIReference
 from ..base import is_complex_attribute
+from ..utils import UNION_TYPES
 from ..utils import normalize_attribute_name
 
 
@@ -117,7 +117,7 @@ def __new__(cls, name, bases, attrs, **kwargs):
             extensions = kwargs["__pydantic_generic_metadata__"]["args"][0]
             extensions = (
                 get_args(extensions)
-                if get_origin(extensions) == Union
+                if get_origin(extensions) in UNION_TYPES
                 else [extensions]
             )
             for extension in extensions:
@@ -183,7 +183,8 @@ def get_extension_models(cls) -> dict[str, type[Extension]]:
         extension_models = cls.__pydantic_generic_metadata__.get("args", [])
         extension_models = (
             get_args(extension_models[0])
-            if len(extension_models) == 1 and get_origin(extension_models[0]) == Union
+            if len(extension_models) == 1
+            and get_origin(extension_models[0]) in UNION_TYPES
             else extension_models
         )
 
@@ -301,7 +302,7 @@ def model_to_schema(model: type[BaseModel]):
 
 def get_reference_types(type) -> list[str]:
     first_arg = get_args(type)[0]
-    types = get_args(first_arg) if get_origin(first_arg) == Union else [first_arg]
+    types = get_args(first_arg) if get_origin(first_arg) in UNION_TYPES else [first_arg]
 
     def serialize_ref_type(ref_type):
         if ref_type == URIReference:
diff --git a/scim2_models/rfc7643/resource_type.py b/scim2_models/rfc7643/resource_type.py
index 17befba..e7b60e3 100644
--- a/scim2_models/rfc7643/resource_type.py
+++ b/scim2_models/rfc7643/resource_type.py
@@ -1,5 +1,7 @@
 from typing import Annotated
 from typing import Optional
+from typing import get_args
+from typing import get_origin
 
 from pydantic import Field
 from typing_extensions import Self
@@ -11,6 +13,7 @@
 from ..base import Required
 from ..base import Returned
 from ..base import URIReference
+from ..utils import UNION_TYPES
 from .resource import Resource
 
 
@@ -82,7 +85,16 @@ def from_resource(cls, resource_model: type[Resource]) -> Self:
         """Build a naive ResourceType from a resource model."""
         schema = resource_model.model_fields["schemas"].default[0]
         name = schema.split(":")[-1]
-        extensions = resource_model.__pydantic_generic_metadata__["args"]
+        if resource_model.__pydantic_generic_metadata__["args"]:
+            extensions = resource_model.__pydantic_generic_metadata__["args"][0]
+            extensions = (
+                get_args(extensions)
+                if get_origin(extensions) in UNION_TYPES
+                else [extensions]
+            )
+        else:
+            extensions = []
+
         return ResourceType(
             id=name,
             name=name,
diff --git a/scim2_models/rfc7644/list_response.py b/scim2_models/rfc7644/list_response.py
index af55ec6..1616890 100644
--- a/scim2_models/rfc7644/list_response.py
+++ b/scim2_models/rfc7644/list_response.py
@@ -20,6 +20,7 @@
 from ..base import Context
 from ..base import Required
 from ..rfc7643.resource import AnyResource
+from ..utils import UNION_TYPES
 from .message import Message
 
 
@@ -29,7 +30,7 @@ def tagged_resource_union(resource_union):
 
         https://docs.pydantic.dev/latest/concepts/unions/#discriminated-unions
         """
-        if not get_origin(resource_union) == Union:
+        if get_origin(resource_union) not in UNION_TYPES:
             return resource_union
 
         resource_types = get_args(resource_union)
diff --git a/tests/test_resource_type.py b/tests/test_resource_type.py
index 2c60fa7..1332bf0 100644
--- a/tests/test_resource_type.py
+++ b/tests/test_resource_type.py
@@ -1,5 +1,10 @@
+from typing import Annotated
+from typing import Union
+
 from scim2_models import EnterpriseUser
+from scim2_models import Extension
 from scim2_models import Reference
+from scim2_models import Required
 from scim2_models import ResourceType
 from scim2_models import User
 
@@ -61,3 +66,32 @@ def test_from_resource_with_extensions():
         == "urn:ietf:params:scim:schemas:extension:enterprise:2.0:User"
     )
     assert not enterprise_user_rt.schema_extensions[0].required
+
+
+def test_from_resource_with_mulitple_extensions():
+    class TestExtension(Extension):
+        schemas: Annotated[list[str], Required.true] = [
+            "urn:ietf:params:scim:schemas:extension:Test:1.0:User"
+        ]
+
+        test: Union[str, None] = None
+        test2: Union[list[str], None] = None
+
+    enterprise_user_rt = ResourceType.from_resource(
+        User[Union[EnterpriseUser, TestExtension]]
+    )
+    assert enterprise_user_rt.id == "User"
+    assert enterprise_user_rt.name == "User"
+    assert enterprise_user_rt.description == "User"
+    assert enterprise_user_rt.endpoint == "/Users"
+    assert enterprise_user_rt.schema_ == "urn:ietf:params:scim:schemas:core:2.0:User"
+    assert (
+        enterprise_user_rt.schema_extensions[0].schema_
+        == "urn:ietf:params:scim:schemas:extension:enterprise:2.0:User"
+    )
+    assert not enterprise_user_rt.schema_extensions[0].required
+    assert (
+        enterprise_user_rt.schema_extensions[1].schema_
+        == "urn:ietf:params:scim:schemas:extension:Test:1.0:User"
+    )
+    assert not enterprise_user_rt.schema_extensions[1].required