diff --git a/kopf/toolkits/hierarchies.py b/kopf/toolkits/hierarchies.py
index 578b8ec..2c54a73 100644
--- a/kopf/toolkits/hierarchies.py
+++ b/kopf/toolkits/hierarchies.py
@@ -2,6 +2,7 @@
 All the functions to properly build the object hierarchies.
 """
 import collections.abc
+import enum
 import warnings
 from typing import Any, Iterable, Iterator, Mapping, MutableMapping, Optional, Union, cast
 
@@ -13,6 +14,10 @@ K8sObject = Union[MutableMapping[Any, Any], thirdparty.PykubeObject, thirdparty.
 K8sObjects = Union[K8sObject, Iterable[K8sObject]]
 
 
+class _UNSET(enum.Enum):
+    token = enum.auto()
+
+
 def append_owner_reference(
         objs: K8sObjects,
         owner: Optional[bodies.Body] = None,
@@ -82,7 +87,7 @@ def remove_owner_reference(
 
 def label(
         objs: K8sObjects,
-        labels: Optional[Mapping[str, Union[None, str]]] = None,
+        labels: Union[Mapping[str, Union[None, str]], _UNSET] = _UNSET.token,
         *,
         forced: bool = False,
         nested: Optional[Union[str, Iterable[dicts.FieldSpec]]] = None,
@@ -97,9 +102,11 @@ def label(
         forced = force
 
     # Try to use the current object being handled if possible.
-    if labels is None:
+    if isinstance(labels, _UNSET):
         real_owner = _guess_owner(None)
         labels = real_owner.get('metadata', {}).get('labels', {})
+    if isinstance(labels, _UNSET):
+        raise RuntimeError("Impossible error: labels are not resolved.")  # for type-checking
 
     # Set labels based on the explicitly specified or guessed ones.
     for obj in cast(Iterator[K8sObject], dicts.walk(objs, nested=nested)):
@@ -124,7 +131,7 @@ def label(
 
 def harmonize_naming(
         objs: K8sObjects,
-        name: Optional[str] = None,
+        name: Union[None, str, _UNSET] = _UNSET.token,
         *,
         forced: bool = False,
         strict: bool = False,
@@ -145,9 +152,11 @@ def harmonize_naming(
     """
 
     # Try to use the current object being handled if possible.
-    if name is None:
+    if isinstance(name, _UNSET):
         real_owner = _guess_owner(None)
         name = real_owner.get('metadata', {}).get('name', None)
+    if isinstance(name, _UNSET):
+        raise RuntimeError("Impossible error: the name is not resolved.")  # for type-checking
     if name is None:
         raise LookupError("Name must be set explicitly: couldn't find it automatically.")
 
@@ -184,7 +193,7 @@ def harmonize_naming(
 
 def adjust_namespace(
         objs: K8sObjects,
-        namespace: Optional[str] = None,
+        namespace: Union[None, str, _UNSET] = _UNSET.token,
         *,
         forced: bool = False,
 ) -> None:
@@ -198,11 +207,11 @@ def adjust_namespace(
     """
 
     # Try to use the current object being handled if possible.
-    if namespace is None:
+    if isinstance(namespace, _UNSET):
         real_owner = _guess_owner(None)
         namespace = real_owner.get('metadata', {}).get('namespace', None)
-    if namespace is None:
-        raise LookupError("Namespace must be set explicitly: couldn't find it automatically.")
+    if isinstance(namespace, _UNSET):
+        raise RuntimeError("Impossible error: the namespace is not resolved.")  # for type-checking
 
     # Set namespace based on the explicitly specified or guessed namespace.
     for obj in cast(Iterator[K8sObject], dicts.walk(objs)):

diff --git a/tests/hierarchies/test_contextual_owner.py b/tests/hierarchies/test_contextual_owner.py
index 0acc456..1138a28 100644
--- a/tests/hierarchies/test_contextual_owner.py
+++ b/tests/hierarchies/test_contextual_owner.py
@@ -108,10 +108,11 @@ def test_when_empty_for_name_harmonization(owner):
 
 
 def test_when_empty_for_namespace_adjustment(owner):
+    # An absent namespace means a cluster-scoped resource -- a valid case.
+    obj = {}
     owner._replace_with({})
-    with pytest.raises(LookupError) as e:
-        kopf.adjust_namespace([])
-    assert 'Namespace must be set explicitly' in str(e.value)
+    kopf.adjust_namespace(obj)
+    assert obj['metadata']['namespace'] is None
 
 
 def test_when_empty_for_adopting(owner):

Regression in kopf.adopt() in 1.30.
## Long story short

The ``kopf.adopt()`` function now only appears to work where parent and child resources are both namespaced. This is not the only combination that is possible, and so code that previous relied on other possibilities now fails.

## Description

For owner references, it is believed the following working combinations work.

* Parent is namespaced, child is namespaced.
* Parent is cluster scoped (non namespaced), child is cluster scoped.
* Parent is cluster scoped, child is namespaced.

Note that the following are believed not to be possible.

* Parent is namespaced, child is namespaced but child is in a different namespace to the parent.
* Parent is namespaced, child is cluster scoped.

Prior to 1.30, all three of the above working combinations were supported. In 1.30, only the first now works.

If you attempt to have the others, you trigger the check:

* https://github.com/nolar/kopf/blob/e71dc607f31ccd0697895e83c3b795c3f685b3d2/kopf/toolkits/hierarchies.py#L204

and get an exception saying that parent needs to be namespaced.

## Environment

* Kopf version: 1.30.2
* Kubernetes version: 1.19
* Python version: 3.7
* OS/platform: Fedora

tests/hierarchies/test_contextual_owner.py::test_when_empty_for_namespace_adjustment[state-changing-cause-namespaced] tests/hierarchies/test_contextual_owner.py::test_when_empty_for_namespace_adjustment[state-changing-cause-cluster] tests/hierarchies/test_contextual_owner.py::test_when_empty_for_namespace_adjustment[event-watching-cause-namespaced] tests/hierarchies/test_contextual_owner.py::test_when_empty_for_namespace_adjustment[event-watching-cause-cluster]
tests/hierarchies/test_contextual_owner.py::test_when_unset_for_owner_references_appending tests/hierarchies/test_contextual_owner.py::test_when_unset_for_owner_references_removal tests/hierarchies/test_contextual_owner.py::test_when_unset_for_name_harmonization tests/hierarchies/test_contextual_owner.py::test_when_unset_for_namespace_adjustment tests/hierarchies/test_contextual_owner.py::test_when_unset_for_labelling tests/hierarchies/test_contextual_owner.py::test_when_unset_for_adopting tests/hierarchies/test_contextual_owner.py::test_when_empty_for_name_harmonization[state-changing-cause-namespaced] tests/hierarchies/test_contextual_owner.py::test_when_empty_for_name_harmonization[state-changing-cause-cluster] tests/hierarchies/test_contextual_owner.py::test_when_empty_for_name_harmonization[event-watching-cause-namespaced] tests/hierarchies/test_contextual_owner.py::test_when_empty_for_name_harmonization[event-watching-cause-cluster] tests/hierarchies/test_contextual_owner.py::test_when_empty_for_adopting[state-changing-cause-namespaced] tests/hierarchies/test_contextual_owner.py::test_when_empty_for_adopting[state-changing-cause-cluster] tests/hierarchies/test_contextual_owner.py::test_when_empty_for_adopting[event-watching-cause-namespaced] tests/hierarchies/test_contextual_owner.py::test_when_empty_for_adopting[event-watching-cause-cluster] tests/hierarchies/test_contextual_owner.py::test_when_set_for_name_harmonization[state-changing-cause-namespaced] tests/hierarchies/test_contextual_owner.py::test_when_set_for_name_harmonization[state-changing-cause-cluster] tests/hierarchies/test_contextual_owner.py::test_when_set_for_name_harmonization[event-watching-cause-namespaced] tests/hierarchies/test_contextual_owner.py::test_when_set_for_name_harmonization[event-watching-cause-cluster] tests/hierarchies/test_contextual_owner.py::test_when_set_for_namespace_adjustment[state-changing-cause-namespaced] tests/hierarchies/test_contextual_owner.py::test_when_set_for_namespace_adjustment[state-changing-cause-cluster] tests/hierarchies/test_contextual_owner.py::test_when_set_for_namespace_adjustment[event-watching-cause-namespaced] tests/hierarchies/test_contextual_owner.py::test_when_set_for_namespace_adjustment[event-watching-cause-cluster] tests/hierarchies/test_contextual_owner.py::test_when_set_for_owner_references_appending[state-changing-cause-namespaced] tests/hierarchies/test_contextual_owner.py::test_when_set_for_owner_references_appending[state-changing-cause-cluster] tests/hierarchies/test_contextual_owner.py::test_when_set_for_owner_references_appending[event-watching-cause-namespaced] tests/hierarchies/test_contextual_owner.py::test_when_set_for_owner_references_appending[event-watching-cause-cluster] tests/hierarchies/test_contextual_owner.py::test_when_set_for_owner_references_removal[state-changing-cause-namespaced] tests/hierarchies/test_contextual_owner.py::test_when_set_for_owner_references_removal[state-changing-cause-cluster] tests/hierarchies/test_contextual_owner.py::test_when_set_for_owner_references_removal[event-watching-cause-namespaced] tests/hierarchies/test_contextual_owner.py::test_when_set_for_owner_references_removal[event-watching-cause-cluster] tests/hierarchies/test_contextual_owner.py::test_when_set_for_labelling[state-changing-cause-namespaced] tests/hierarchies/test_contextual_owner.py::test_when_set_for_labelling[state-changing-cause-cluster] tests/hierarchies/test_contextual_owner.py::test_when_set_for_labelling[event-watching-cause-namespaced] tests/hierarchies/test_contextual_owner.py::test_when_set_for_labelling[event-watching-cause-cluster]