diff --git a/CHANGES b/CHANGES
index 67ef71dd7..143e236a4 100644
--- a/CHANGES
+++ b/CHANGES
@@ -10,6 +10,8 @@ Incompatible changes
 * #9222: Update Underscore.js to 1.13.1
 * #9217: manpage: Stop creating a section directory on build manpage by default
   (see :confval:`man_make_section_directory`)
+* #9240: Unknown node error for pending_xref_condition is raised if an extension
+  that does not support the node installs a missing-reference handler
 
 Deprecated
 ----------
diff --git a/sphinx/transforms/post_transforms/__init__.py b/sphinx/transforms/post_transforms/__init__.py
index e2899d994..dde06fae1 100644
--- a/sphinx/transforms/post_transforms/__init__.py
+++ b/sphinx/transforms/post_transforms/__init__.py
@@ -8,10 +8,10 @@
     :license: BSD, see LICENSE for details.
 """
 
-from typing import Any, Dict, List, Optional, Tuple, Type, cast
+from typing import Any, Dict, List, Optional, Sequence, Tuple, Type, cast
 
 from docutils import nodes
-from docutils.nodes import Element
+from docutils.nodes import Element, Node
 
 from sphinx import addnodes
 from sphinx.addnodes import pending_xref
@@ -26,10 +26,6 @@ from sphinx.util.nodes import find_pending_xref_condition, process_only_nodes
 
 logger = logging.getLogger(__name__)
 
-if False:
-    # For type annotation
-    from docutils.nodes import Node
-
 
 class SphinxPostTransform(SphinxTransform):
     """A base class of post-transforms.
@@ -71,7 +67,12 @@ class ReferencesResolver(SphinxPostTransform):
 
     def run(self, **kwargs: Any) -> None:
         for node in self.document.traverse(addnodes.pending_xref):
-            contnode = cast(nodes.TextElement, node[0].deepcopy())
+            content = self.find_pending_xref_condition(node, ("resolved", "*"))
+            if content:
+                contnode = cast(Element, content[0].deepcopy())
+            else:
+                contnode = cast(Element, node[0].deepcopy())
+
             newnode = None
 
             typ = node['reftype']
@@ -108,9 +109,9 @@ class ReferencesResolver(SphinxPostTransform):
             else:
                 newnodes = [contnode]
                 if newnode is None and isinstance(node[0], addnodes.pending_xref_condition):
-                    matched = find_pending_xref_condition(node, "*")
+                    matched = self.find_pending_xref_condition(node, ("*",))
                     if matched:
-                        newnodes = matched.children
+                        newnodes = matched
                     else:
                         logger.warning(__('Could not determine the fallback text for the '
                                           'cross-reference. Might be a bug.'), location=node)
@@ -193,6 +194,15 @@ class ReferencesResolver(SphinxPostTransform):
             msg = __('%r reference target not found: %s') % (typ, target)
         logger.warning(msg, location=node, type='ref', subtype=typ)
 
+    def find_pending_xref_condition(self, node: pending_xref, conditions: Sequence[str]
+                                    ) -> Optional[List[Node]]:
+        for condition in conditions:
+            matched = find_pending_xref_condition(node, condition)
+            if matched:
+                return matched.children
+        else:
+            return None
+
 
 class OnlyNodeTransform(SphinxPostTransform):
     default_priority = 50
diff --git a/tests/roots/test-transforms-post_transforms-missing-reference/conf.py b/tests/roots/test-transforms-post_transforms-missing-reference/conf.py
new file mode 100644
index 000000000..2db221cc6
--- /dev/null
+++ b/tests/roots/test-transforms-post_transforms-missing-reference/conf.py
@@ -0,0 +1 @@
+nitpicky = True
diff --git a/tests/roots/test-transforms-post_transforms-missing-reference/index.rst b/tests/roots/test-transforms-post_transforms-missing-reference/index.rst
new file mode 100644
index 000000000..7180978b2
--- /dev/null
+++ b/tests/roots/test-transforms-post_transforms-missing-reference/index.rst
@@ -0,0 +1,5 @@
+transforms-post_transforms-missing-reference
+============================================
+
+:class:`io.StringIO`
+
diff --git a/tests/test_transforms_post_transforms.py b/tests/test_transforms_post_transforms.py
new file mode 100644
index 000000000..26af55031
--- /dev/null
+++ b/tests/test_transforms_post_transforms.py
@@ -0,0 +1,58 @@
+"""
+    test_transforms_post_transforms
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+    Tests the post_transforms
+
+    :copyright: Copyright 2007-2021 by the Sphinx team, see AUTHORS.
+    :license: BSD, see LICENSE for details.
+"""
+
+import pytest
+from docutils import nodes
+
+
+@pytest.mark.sphinx('html', testroot='transforms-post_transforms-missing-reference')
+def test_nitpicky_warning(app, status, warning):
+    app.build()
+    assert ('index.rst:4: WARNING: py:class reference target '
+            'not found: io.StringIO' in warning.getvalue())
+
+    content = (app.outdir / 'index.html').read_text()
+    assert ('<p><code class="xref py py-class docutils literal notranslate"><span class="pre">'
+            'io.StringIO</span></code></p>' in content)
+
+
+@pytest.mark.sphinx('html', testroot='transforms-post_transforms-missing-reference',
+                    freshenv=True)
+def test_missing_reference(app, status, warning):
+    def missing_reference(app, env, node, contnode):
+        assert app is app
+        assert env is app.env
+        assert node['reftarget'] == 'io.StringIO'
+        assert contnode.astext() == 'io.StringIO'
+
+        return nodes.inline('', 'missing-reference.StringIO')
+
+    warning.truncate(0)
+    app.connect('missing-reference', missing_reference)
+    app.build()
+    assert warning.getvalue() == ''
+
+    content = (app.outdir / 'index.html').read_text()
+    assert '<p><span>missing-reference.StringIO</span></p>' in content
+
+
+@pytest.mark.sphinx('html', testroot='domain-py-python_use_unqualified_type_names',
+                    freshenv=True)
+def test_missing_reference_conditional_pending_xref(app, status, warning):
+    def missing_reference(app, env, node, contnode):
+        return contnode
+
+    warning.truncate(0)
+    app.connect('missing-reference', missing_reference)
+    app.build()
+    assert warning.getvalue() == ''
+
+    content = (app.outdir / 'index.html').read_text()
+    assert '<span class="n"><span class="pre">Age</span></span>' in content