diff --git a/singer_sdk/helpers/_typing.py b/singer_sdk/helpers/_typing.py
index b9beca6cd..db4c88820 100644
--- a/singer_sdk/helpers/_typing.py
+++ b/singer_sdk/helpers/_typing.py
@@ -80,10 +80,12 @@ def append_type(type_dict: dict, new_type: str) -> dict:
     """Return a combined type definition using the 'anyOf' JSON Schema construct."""
     result = copy.deepcopy(type_dict)
     if "anyOf" in result:
-        if isinstance(result["anyOf"], list) and new_type not in result["anyOf"]:
-            result["anyOf"].append(new_type)
-        elif new_type != result["anyOf"]:
-            result["anyOf"] = [result["anyOf"], new_type]
+        new_type_dict = {"type": new_type}
+        if isinstance(result["anyOf"], list):
+            if new_type_dict not in result["anyOf"]:
+                result["anyOf"].append(new_type_dict)
+        elif new_type_dict != result["anyOf"]:
+            result["anyOf"] = [result["anyOf"], new_type_dict]
         return result
 
     if "oneOf" in result:
diff --git a/tests/core/test_jsonschema_helpers.py b/tests/core/test_jsonschema_helpers.py
index 28ce1c3fd..5868d3016 100644
--- a/tests/core/test_jsonschema_helpers.py
+++ b/tests/core/test_jsonschema_helpers.py
@@ -29,6 +29,7 @@
 from singer_sdk.typing import (
     DEFAULT_JSONSCHEMA_VALIDATOR,
     AllOf,
+    AnyOf,
     AnyType,
     ArrayType,
     BooleanType,
@@ -49,6 +50,7 @@
     IRIType,
     JSONPointerType,
     ObjectType,
+    OneOf,
     PropertiesList,
     Property,
     RegexType,
@@ -641,9 +643,10 @@ def test_property_creation(
 ) -> None:
     property_dict = property_obj.to_dict()
     assert property_dict == expected_jsonschema
+    property_name = next(iter(property_dict.keys()))
+    property_node = property_dict[property_name]
+    DEFAULT_JSONSCHEMA_VALIDATOR.check_schema(property_node)
     for check_fn in TYPE_FN_CHECKS:
-        property_name = next(iter(property_dict.keys()))
-        property_node = property_dict[property_name]
         if check_fn in type_fn_checks_true:
             assert check_fn(property_node) is True, (
                 f"{check_fn.__name__} was not True for {property_dict!r}"
@@ -654,6 +657,77 @@ def test_property_creation(
             )
 
 
+@pytest.mark.parametrize(
+    "property_obj,expected_jsonschema",
+    [
+        pytest.param(
+            Property(
+                "my_prop",
+                AnyOf(StringType, ArrayType(StringType)),
+                required=True,
+            ),
+            {
+                "my_prop": {
+                    "anyOf": [
+                        {"type": ["string"]},
+                        {"type": "array", "items": {"type": ["string"]}},
+                    ],
+                },
+            },
+            id="anyof_required",
+        ),
+        pytest.param(
+            Property(
+                "my_prop",
+                AnyOf(StringType, ArrayType(StringType)),
+                required=False,
+            ),
+            {
+                "my_prop": {
+                    "anyOf": [
+                        {"type": ["string"]},
+                        {"type": "array", "items": {"type": ["string"]}},
+                        {"type": "null"},
+                    ],
+                },
+            },
+            id="anyof_optional",
+        ),
+        pytest.param(
+            Property(
+                "my_prop",
+                OneOf(StringType, IntegerType),
+                required=False,
+            ),
+            {
+                "my_prop": {
+                    "oneOf": [
+                        {"type": ["string"]},
+                        {"type": ["integer"]},
+                        {"type": "null"},
+                    ],
+                },
+            },
+            id="oneof_optional",
+        ),
+    ],
+)
+def test_optional_composite_type_produces_valid_schema(
+    property_obj: Property,
+    expected_jsonschema: dict,
+) -> None:
+    """Optional `AnyOf`/`OneOf` properties must stay valid JSON Schema.
+
+    Regression test: `append_type` used to splice the bare string "null" into
+    an existing `anyOf`/`oneOf` list instead of a `{"type": "null"}` subschema,
+    which fails JSON Schema meta-validation.
+    """
+    property_dict = property_obj.to_dict()
+    assert property_dict == expected_jsonschema
+    property_name = next(iter(property_dict.keys()))
+    DEFAULT_JSONSCHEMA_VALIDATOR.check_schema(property_dict[property_name])
+
+
 def test_wrapped_type_dict():
     with pytest.raises(
         ValueError,
diff --git a/tests/core/test_typing.py b/tests/core/test_typing.py
index eaf4da9b2..6bd4c6219 100644
--- a/tests/core/test_typing.py
+++ b/tests/core/test_typing.py
@@ -417,7 +417,7 @@ def test_conform_primitives(value: t.Any, type_dict: dict, expected: t.Any):
         pytest.param({"type": "array"}, {"type": ["array", "null"]}, id="array"),
         pytest.param(
             {"anyOf": [{"type": "integer"}, {"type": "number"}]},
-            {"anyOf": [{"type": "integer"}, {"type": "number"}, "null"]},
+            {"anyOf": [{"type": "integer"}, {"type": "number"}, {"type": "null"}]},
             id="anyOf",
         ),
         pytest.param(