diff --git a/Include/internal/pycore_global_objects_fini_generated.h b/Include/internal/pycore_global_objects_fini_generated.h
index 4b1e289c6ff468..91bf3fcbc0eced 100644
--- a/Include/internal/pycore_global_objects_fini_generated.h
+++ b/Include/internal/pycore_global_objects_fini_generated.h
@@ -1576,6 +1576,7 @@ _PyStaticObjects_CheckRefcnt(PyInterpreterState *interp) {
     _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(aclose));
     _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(add));
     _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(add_done_callback));
+    _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(add_note));
     _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(adobe));
     _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(after_in_child));
     _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(after_in_parent));
diff --git a/Include/internal/pycore_global_strings.h b/Include/internal/pycore_global_strings.h
index 6ee649b59a5c37..73804a531db224 100644
--- a/Include/internal/pycore_global_strings.h
+++ b/Include/internal/pycore_global_strings.h
@@ -299,6 +299,7 @@ struct _Py_global_strings {
         STRUCT_FOR_ID(aclose)
         STRUCT_FOR_ID(add)
         STRUCT_FOR_ID(add_done_callback)
+        STRUCT_FOR_ID(add_note)
         STRUCT_FOR_ID(adobe)
         STRUCT_FOR_ID(after_in_child)
         STRUCT_FOR_ID(after_in_parent)
diff --git a/Include/internal/pycore_runtime_init_generated.h b/Include/internal/pycore_runtime_init_generated.h
index 778db946c2a3aa..25c0fde5657fc0 100644
--- a/Include/internal/pycore_runtime_init_generated.h
+++ b/Include/internal/pycore_runtime_init_generated.h
@@ -1574,6 +1574,7 @@ extern "C" {
     INIT_ID(aclose), \
     INIT_ID(add), \
     INIT_ID(add_done_callback), \
+    INIT_ID(add_note), \
     INIT_ID(adobe), \
     INIT_ID(after_in_child), \
     INIT_ID(after_in_parent), \
diff --git a/Include/internal/pycore_unicodeobject_generated.h b/Include/internal/pycore_unicodeobject_generated.h
index bd8f50ff0ee732..ab448dcc1e5853 100644
--- a/Include/internal/pycore_unicodeobject_generated.h
+++ b/Include/internal/pycore_unicodeobject_generated.h
@@ -976,6 +976,10 @@ _PyUnicode_InitStaticStrings(PyInterpreterState *interp) {
     _PyUnicode_InternStatic(interp, &string);
     assert(_PyUnicode_CheckConsistency(string, 1));
     assert(PyUnicode_GET_LENGTH(string) != 1);
+    string = &_Py_ID(add_note);
+    _PyUnicode_InternStatic(interp, &string);
+    assert(_PyUnicode_CheckConsistency(string, 1));
+    assert(PyUnicode_GET_LENGTH(string) != 1);
     string = &_Py_ID(adobe);
     _PyUnicode_InternStatic(interp, &string);
     assert(_PyUnicode_CheckConsistency(string, 1));
diff --git a/Lib/test/test_pyexpat.py b/Lib/test/test_pyexpat.py
index cace780f79f515..aaa91aca36e3c4 100644
--- a/Lib/test/test_pyexpat.py
+++ b/Lib/test/test_pyexpat.py
@@ -510,6 +510,34 @@ def _test_exception(self, have_source):
             self.assertIn('call_with_frame("StartElement"',
                           entries[1].line)
 
+    def test_invalid_NotStandalone(self):
+        parser = expat.ParserCreate()
+        parser.NotStandaloneHandler = mock.Mock(return_value="bad value")
+        parser.ElementDeclHandler = lambda _1, _2: None
+
+        payload = b"""\
+<!DOCTYPE quotations SYSTEM "quotations.dtd" [<!ELEMENT root ANY>]><root/>
+"""
+        with self.assertRaises(TypeError) as cm:
+            parser.Parse(payload, True)
+        parser.NotStandaloneHandler.assert_called_once()
+
+        notes = ["invalid 'NotStandalone' event handler return value"]
+        self.assertEqual(cm.exception.__notes__, notes)
+
+    def test_invalid_ExternalEntityRefHandler(self):
+        parser = expat.ParserCreate()
+        parser.UseForeignDTD()
+        parser.SetParamEntityParsing(expat.XML_PARAM_ENTITY_PARSING_ALWAYS)
+        parser.ExternalEntityRefHandler = mock.Mock(return_value=None)
+
+        with self.assertRaises(TypeError) as cm:
+            parser.Parse(b"<?xml version='1.0'?><element/>", True)
+        parser.ExternalEntityRefHandler.assert_called_once()
+
+        notes = ["invalid 'ExternalEntityRef' event handler return value"]
+        self.assertEqual(cm.exception.__notes__, notes)
+
 
 # Test Current* members:
 class PositionTest(unittest.TestCase):
diff --git a/Misc/NEWS.d/next/Library/2026-03-28-11-31-32.gh-issue-146563.cXtSym.rst b/Misc/NEWS.d/next/Library/2026-03-28-11-31-32.gh-issue-146563.cXtSym.rst
new file mode 100644
index 00000000000000..2103024b616d4e
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-03-28-11-31-32.gh-issue-146563.cXtSym.rst
@@ -0,0 +1,2 @@
+:mod:`xml.parsers.expat`: add an exception note when a custom Expat handler
+return value cannot be properly interpreted. Patch by Bénédikt Tran.
diff --git a/Modules/pyexpat.c b/Modules/pyexpat.c
index 31b883fe8bd548..e915054143c51d 100644
--- a/Modules/pyexpat.c
+++ b/Modules/pyexpat.c
@@ -503,6 +503,29 @@ my_StartElementHandler(void *userData,
     }
 }
 
+static inline void
+invalid_expat_handler_rv(const char *name)
+{
+    PyObject *exc = PyErr_GetRaisedException();
+    assert(exc != NULL);
+    PyObject *note = PyUnicode_FromFormat("invalid '%s' event handler return value", name);
+    if (note == NULL) {
+        goto error;
+    }
+    PyObject *res = PyObject_CallMethodOneArg(exc, &_Py_ID(add_note), note);
+    Py_DECREF(note);
+    if (res == NULL) {
+        goto error;
+    }
+    Py_DECREF(res);
+    goto done;
+
+error:
+    PyErr_Clear();
+done:
+    PyErr_SetRaisedException(exc);
+}
+
 #define RC_HANDLER(RETURN_TYPE, NAME, PARAMS,       \
                    INIT, PARSE_FORMAT, CONVERSION,  \
                    RETURN_VARIABLE, GETUSERDATA)    \
@@ -536,6 +559,9 @@ my_ ## NAME ## Handler PARAMS {                     \
     }                                               \
     CONVERSION                                      \
     Py_DECREF(rv);                                  \
+    if (PyErr_Occurred()) {                         \
+        invalid_expat_handler_rv(#NAME);            \
+    }                                               \
     return RETURN_VARIABLE;                         \
 }