diff --git a/Lib/shelve.py b/Lib/shelve.py
index 1010be1e09d702..63e4f116b52c4a 100644
--- a/Lib/shelve.py
+++ b/Lib/shelve.py
@@ -106,6 +106,17 @@ def __init__(self, dict, protocol=None, writeback=False,
             self.serializer = serializer
             self.deserializer = deserializer
 
+    @staticmethod
+    def _validate_serialized_value(serialized_value, original_value):
+        if (serialized_value is None or
+            not isinstance(serialized_value, (bytes, str))):
+            if serialized_value is None:
+                invalid_type = "None"
+            else:
+                invalid_type = type(serialized_value).__name__
+            msg = f"Serializer must return bytes or str, not {invalid_type}"
+            raise ShelveError(msg)
+
     def __iter__(self):
         for k in self.dict.keys():
             yield k.decode(self.keyencoding)
@@ -135,6 +146,7 @@ def __setitem__(self, key, value):
         if self.writeback:
             self.cache[key] = value
         serialized_value = self.serializer(value, self._protocol)
+        self._validate_serialized_value(serialized_value, value)
         self.dict[key.encode(self.keyencoding)] = serialized_value
 
     def __delitem__(self, key):
diff --git a/Lib/test/test_dbm_gnu.py b/Lib/test/test_dbm_gnu.py
index 66268c42a300b5..c092357d92a635 100644
--- a/Lib/test/test_dbm_gnu.py
+++ b/Lib/test/test_dbm_gnu.py
@@ -217,6 +217,25 @@ def test_localized_error(self):
             create_empty_file(os.path.join(d, 'test'))
             self.assertRaises(gdbm.error, gdbm.open, filename, 'r')
 
+    def test_type_errors(self):
+        self.g = gdbm.open(filename, 'c')
+        with self.assertRaisesRegex(
+            TypeError, "^a bytes-like object is required, not 'int'$",
+        ):
+            self.g[123]
+        with self.assertRaisesRegex(
+            TypeError, "^gdbm key must be bytes or str, not 'int'$",
+        ):
+            123 in self.g
+        with self.assertRaisesRegex(
+            TypeError, "^gdbm key must be bytes or str, not 'NoneType'$",
+        ):
+            self.g[None] = 123
+        with self.assertRaisesRegex(
+            TypeError, "^gdbm value must be bytes or str, not 'int'$",
+        ):
+            self.g['foo'] = 123
+
 
 if __name__ == '__main__':
     unittest.main()
diff --git a/Lib/test/test_dbm_ndbm.py b/Lib/test/test_dbm_ndbm.py
index e0f31c9a9a337d..01da376b599b3d 100644
--- a/Lib/test/test_dbm_ndbm.py
+++ b/Lib/test/test_dbm_ndbm.py
@@ -160,6 +160,25 @@ def test_clear(self):
                 self.assertNotIn(k, db)
             self.assertEqual(len(db), 0)
 
+    def test_type_errors(self):
+        with dbm.ndbm.open(self.filename, 'c') as db:
+            with self.assertRaisesRegex(
+                TypeError, "^a bytes-like object is required, not 'int'$",
+            ):
+                db[123]
+            with self.assertRaisesRegex(
+                TypeError, "^dbm key must be bytes or str, not 'int'$",
+            ):
+                123 in db
+            with self.assertRaisesRegex(
+                TypeError, "^dbm key must be bytes or str, not 'NoneType'$",
+            ):
+                db[None] = 123
+            with self.assertRaisesRegex(
+                TypeError, "^dbm value must be bytes or str, not 'int'$",
+            ):
+                db['foo'] = 123
+
 
 if __name__ == '__main__':
     unittest.main()
diff --git a/Lib/test/test_shelve.py b/Lib/test/test_shelve.py
index 64609ab9dd9a62..931f1ed8e6147f 100644
--- a/Lib/test/test_shelve.py
+++ b/Lib/test/test_shelve.py
@@ -173,6 +173,8 @@ def test_custom_serializer_and_deserializer(self):
         def serializer(obj, protocol):
             if isinstance(obj, (bytes, bytearray, str)):
                 if protocol == 5:
+                    if isinstance(obj, bytearray):
+                        return bytes(obj)  # DBM backends expect bytes
                     return obj
                 return type(obj).__name__
             elif isinstance(obj, array.array):
@@ -223,11 +225,10 @@ def deserializer(data):
                     )
 
     def test_custom_incomplete_serializer_and_deserializer(self):
-        dbm_sqlite3 = import_helper.import_module("dbm.sqlite3")
         os.mkdir(self.dirname)
         self.addCleanup(os_helper.rmtree, self.dirname)
 
-        with self.assertRaises(dbm_sqlite3.error):
+        with self.assertRaises(shelve.ShelveError):
             def serializer(obj, protocol=None):
                 pass
 
@@ -430,6 +431,79 @@ def setUp(self):
         dbm._defaultmod = self.dbm_mod
 
 
+class TestShelveValidation(unittest.TestCase):
+    dirname = os_helper.TESTFN
+    fname = os.path.join(dirname, os_helper.TESTFN)
+
+    def setup_test_dir(self):
+        os_helper.rmtree(self.dirname)
+        os.mkdir(self.dirname)
+
+    def setUp(self):
+        self.addCleanup(setattr, dbm, "_defaultmod", dbm._defaultmod)
+        os.mkdir(self.dirname)
+        self.addCleanup(os_helper.rmtree, self.dirname)
+
+    def test_serializer_unsupported_return_type(self):
+        def int_serializer(obj, protocol=None):
+            return 3
+
+        def none_serializer(obj, protocol=None):
+            return None
+
+        def deserializer(data):
+            if isinstance(data, bytes):
+                return data.decode("utf-8")
+            else:
+                return data
+
+        for module in dbm_iterator():
+            self.setup_test_dir()
+            dbm._defaultmod = module
+            with module.open(self.fname, "c"):
+                pass
+            self.assertEqual(module.__name__, dbm.whichdb(self.fname))
+
+            with shelve.open(self.fname, serializer=none_serializer,
+                             deserializer=deserializer) as s:
+                with self.assertRaises(shelve.ShelveError) as cm:
+                    s["key"] = "value"
+                self.assertEqual("Serializer must return bytes or str, not None",
+                                 f"{cm.exception}")
+
+            with shelve.open(self.fname, serializer=int_serializer,
+                             deserializer=deserializer,) as s:
+                with self.assertRaises(shelve.ShelveError) as cm:
+                    s["key"] = "value"
+                self.assertEqual("Serializer must return bytes or str, not int",
+                                 f"{cm.exception}")
+
+    def test_shelve_type_compatibility(self):
+        for module in dbm_iterator():
+            self.setup_test_dir()
+            dbm._defaultmod = module
+            with shelve.Shelf(module.open(self.fname, "c")) as shelf:
+                shelf["string"] = "hello"
+                shelf["bytes"] = b"world"
+                shelf["number"] = 42
+                shelf["list"] = [1, 2, 3]
+                shelf["dict"] = {"key": "value"}
+                shelf["set"] = {1, 2, 3}
+                shelf["tuple"] = (1, 2, 3)
+                shelf["complex"] = 1 + 2j
+                shelf["bytearray"] = bytearray(b"test")
+                shelf["array"] = array.array("i", [1, 2, 3])
+                self.assertEqual(shelf["string"], "hello")
+                self.assertEqual(shelf["bytes"], b"world")
+                self.assertEqual(shelf["number"], 42)
+                self.assertEqual(shelf["list"], [1, 2, 3])
+                self.assertEqual(shelf["dict"], {"key": "value"})
+                self.assertEqual(shelf["set"], {1, 2, 3})
+                self.assertEqual(shelf["tuple"], (1, 2, 3))
+                self.assertEqual(shelf["complex"], 1 + 2j)
+                self.assertEqual(shelf["bytearray"], bytearray(b"test"))
+                self.assertEqual(shelf["array"], array.array("i", [1, 2, 3]))
+
 from test import mapping_tests
 
 for proto in range(pickle.HIGHEST_PROTOCOL + 1):
diff --git a/Misc/NEWS.d/next/Library/2025-11-16-15-29-49.gh-issue-137899.JnbEmT.rst b/Misc/NEWS.d/next/Library/2025-11-16-15-29-49.gh-issue-137899.JnbEmT.rst
new file mode 100644
index 00000000000000..12faf24a0a6d46
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-11-16-15-29-49.gh-issue-137899.JnbEmT.rst
@@ -0,0 +1 @@
+The :mod:`shelve` module will now provide descriptive error messages to better distinguish key and value type errors. Patch by Furkan Onder.
diff --git a/Modules/_dbmmodule.c b/Modules/_dbmmodule.c
index f88861fa24423b..2b0a73a22d3c6c 100644
--- a/Modules/_dbmmodule.c
+++ b/Modules/_dbmmodule.c
@@ -231,8 +231,9 @@ dbm_ass_sub_lock_held(PyObject *self, PyObject *v, PyObject *w)
     dbmobject *dp = dbmobject_CAST(self);
 
     if ( !PyArg_Parse(v, "s#", &krec.dptr, &tmp_size) ) {
-        PyErr_SetString(PyExc_TypeError,
-                        "dbm mappings have bytes or string keys only");
+        PyErr_Format(PyExc_TypeError,
+                     "dbm key must be bytes or str, not '%T'",
+                     v);
         return -1;
     }
     _dbm_state *state = PyType_GetModuleState(Py_TYPE(dp));
@@ -258,8 +259,9 @@ dbm_ass_sub_lock_held(PyObject *self, PyObject *v, PyObject *w)
         }
     } else {
         if ( !PyArg_Parse(w, "s#", &drec.dptr, &tmp_size) ) {
-            PyErr_SetString(PyExc_TypeError,
-                 "dbm mappings have bytes or string elements only");
+            PyErr_Format(PyExc_TypeError,
+                         "dbm value must be bytes or str, not '%T'",
+                         w);
             return -1;
         }
         drec.dsize = tmp_size;
@@ -369,8 +371,7 @@ dbm_contains_lock_held(PyObject *self, PyObject *arg)
     }
     else if (!PyBytes_Check(arg)) {
         PyErr_Format(PyExc_TypeError,
-                     "dbm key must be bytes or string, not %.100s",
-                     Py_TYPE(arg)->tp_name);
+                     "dbm key must be bytes or str, not '%T'", arg);
         return -1;
     }
     else {
diff --git a/Modules/_gdbmmodule.c b/Modules/_gdbmmodule.c
index 72f568ceb06987..b1bbcc86df5892 100644
--- a/Modules/_gdbmmodule.c
+++ b/Modules/_gdbmmodule.c
@@ -237,12 +237,14 @@ gdbm_bool(PyObject *op)
 // This function is needed to support PY_SSIZE_T_CLEAN.
 // Return 1 on success, same to PyArg_Parse().
 static int
-parse_datum(PyObject *o, datum *d, const char *failmsg)
+parse_datum(PyObject *o, datum *d, const char *item_name)
 {
     Py_ssize_t size;
     if (!PyArg_Parse(o, "s#", &d->dptr, &size)) {
-        if (failmsg != NULL) {
-            PyErr_SetString(PyExc_TypeError, failmsg);
+        if (item_name) {
+            PyErr_Format(PyExc_TypeError,
+                         "gdbm %s must be bytes or str, not '%T'",
+                         item_name, o);
         }
         return 0;
     }
@@ -318,11 +320,10 @@ static int
 gdbm_ass_sub_lock_held(PyObject *op, PyObject *v, PyObject *w)
 {
     datum krec, drec;
-    const char *failmsg = "gdbm mappings have bytes or string indices only";
     gdbmobject *dp = _gdbmobject_CAST(op);
     _gdbm_state *state = PyType_GetModuleState(Py_TYPE(dp));
 
-    if (!parse_datum(v, &krec, failmsg)) {
+    if (!parse_datum(v, &krec, "key")) {
         return -1;
     }
     if (dp->di_dbm == NULL) {
@@ -343,7 +344,7 @@ gdbm_ass_sub_lock_held(PyObject *op, PyObject *v, PyObject *w)
         }
     }
     else {
-        if (!parse_datum(w, &drec, failmsg)) {
+        if (!parse_datum(w, &drec, "value")) {
             return -1;
         }
         errno = 0;
@@ -491,8 +492,7 @@ gdbm_contains_lock_held(PyObject *self, PyObject *arg)
     }
     else if (!PyBytes_Check(arg)) {
         PyErr_Format(PyExc_TypeError,
-                     "gdbm key must be bytes or string, not %.100s",
-                     Py_TYPE(arg)->tp_name);
+                     "gdbm key must be bytes or str, not '%T'", arg);
         return -1;
     }
     else {