diff --git a/src/cattrs/converters.py b/src/cattrs/converters.py
index 16e74ed8..2559bca1 100644
--- a/src/cattrs/converters.py
+++ b/src/cattrs/converters.py
@@ -963,7 +963,6 @@ def _get_dis_func(
             # logic.
             union_types = tuple(e for e in union_types if e is not NoneType)
 
-        # TODO: technically both disambiguators could support TypedDicts too
         if not all(has(get_origin(e) or e) for e in union_types):
             raise StructureHandlerNotFoundError(
                 "Only unions of attrs classes and dataclasses supported "
diff --git a/src/cattrs/disambiguators.py b/src/cattrs/disambiguators.py
index 83e8c3f1..80288024 100644
--- a/src/cattrs/disambiguators.py
+++ b/src/cattrs/disambiguators.py
@@ -30,7 +30,7 @@
 
 
 def is_supported_union(typ: Any) -> bool:
-    """Whether the type is a union of attrs classes."""
+    """Whether the type is a union of attrs classes or dataclasses."""
     return is_union_type(typ) and all(
         e is NoneType or has(get_origin(e) or e) for e in typ.__args__
     )
diff --git a/src/cattrs/strategies/_subclasses.py b/src/cattrs/strategies/_subclasses.py
index 06a92afa..47f3e7de 100644
--- a/src/cattrs/strategies/_subclasses.py
+++ b/src/cattrs/strategies/_subclasses.py
@@ -84,7 +84,7 @@ def include_subclasses(
 def _include_subclasses_without_union_strategy(
     cl,
     converter: BaseConverter,
-    parent_subclass_tree: tuple[type],
+    parent_subclass_tree: tuple[type, ...],
     overrides: dict[str, AttributeOverride] | None,
 ):
     # The iteration approach is required if subclasses are more than one level deep:
diff --git a/tests/strategies/test_include_subclasses.py b/tests/strategies/test_include_subclasses.py
index 7b6b9861..02746305 100644
--- a/tests/strategies/test_include_subclasses.py
+++ b/tests/strategies/test_include_subclasses.py
@@ -1,13 +1,12 @@
 import typing
 from copy import deepcopy
 from functools import partial
-from typing import List, Tuple
 
 import pytest
 from attrs import define
 
 from cattrs import Converter, override
-from cattrs.errors import ClassValidationError
+from cattrs.errors import ClassValidationError, StructureHandlerNotFoundError
 from cattrs.strategies import configure_tagged_union, include_subclasses
 
 
@@ -148,7 +147,7 @@ def conv_w_subclasses(request):
     "struct_unstruct", IDS_TO_STRUCT_UNSTRUCT.values(), ids=IDS_TO_STRUCT_UNSTRUCT
 )
 def test_structuring_with_inheritance(
-    conv_w_subclasses: Tuple[Converter, bool], struct_unstruct
+    conv_w_subclasses: tuple[Converter, bool], struct_unstruct
 ) -> None:
     structured, unstructured = struct_unstruct
 
@@ -219,7 +218,7 @@ def test_circular_reference(conv_w_subclasses):
     "struct_unstruct", IDS_TO_STRUCT_UNSTRUCT.values(), ids=IDS_TO_STRUCT_UNSTRUCT
 )
 def test_unstructuring_with_inheritance(
-    conv_w_subclasses: Tuple[Converter, bool], struct_unstruct
+    conv_w_subclasses: tuple[Converter, bool], struct_unstruct
 ):
     structured, unstructured = struct_unstruct
     converter, included_subclasses_param = conv_w_subclasses
@@ -389,5 +388,27 @@ class Derived(A):
                 "_type": "Derived",
             }
         ],
-        List[A],
+        list[A],
     ) == [Derived(9, Derived(99, A(999)))]
+
+
+def test_unsupported_class(genconverter: Converter):
+    """Non-attrs/dataclass classes raise proper errors."""
+
+    class NewParent:
+        """Not an attrs class."""
+
+        a: int
+
+    @define
+    class NewChild(NewParent):
+        pass
+
+    @define
+    class NewChild2(NewParent):
+        pass
+
+    genconverter.register_structure_hook(NewParent, lambda v, _: NewParent(v))
+
+    with pytest.raises(StructureHandlerNotFoundError):
+        include_subclasses(NewParent, genconverter)