diff --git a/Lib/test/test_getargs2.py b/Lib/test/test_getargs2.py
index be777af6b4b..6e2fa79276a 100644
--- a/Lib/test/test_getargs2.py
+++ b/Lib/test/test_getargs2.py
@@ -70,6 +70,12 @@ class BadInt3(int):
     def __int__(self):
         return True
 
+class TupleSubclass(tuple):
+    pass
+
+class DictSubclass(dict):
+    pass
+
 
 class Unsigned_TestCase(unittest.TestCase):
     def test_b(self):
@@ -321,6 +327,33 @@ class Boolean_TestCase(unittest.TestCase):
 
 
 class Tuple_TestCase(unittest.TestCase):
+    def test_args(self):
+        from _testcapi import get_args
+
+        ret = get_args(1, 2)
+        self.assertEqual(ret, (1, 2))
+        self.assertIs(type(ret), tuple)
+
+        ret = get_args(1, *(2, 3))
+        self.assertEqual(ret, (1, 2, 3))
+        self.assertIs(type(ret), tuple)
+
+        ret = get_args(*[1, 2])
+        self.assertEqual(ret, (1, 2))
+        self.assertIs(type(ret), tuple)
+
+        ret = get_args(*TupleSubclass([1, 2]))
+        self.assertEqual(ret, (1, 2))
+        self.assertIs(type(ret), tuple)
+
+        ret = get_args()
+        self.assertIn(ret, ((), None))
+        self.assertIn(type(ret), (tuple, type(None)))
+
+        ret = get_args(*())
+        self.assertIn(ret, ((), None))
+        self.assertIn(type(ret), (tuple, type(None)))
+
     def test_tuple(self):
         from _testcapi import getargs_tuple
 
@@ -336,6 +369,29 @@ class Tuple_TestCase(unittest.TestCase):
         self.assertRaises(TypeError, getargs_tuple, 1, seq())
 
 class Keywords_TestCase(unittest.TestCase):
+    def test_kwargs(self):
+        from _testcapi import get_kwargs
+
+        ret = get_kwargs(a=1, b=2)
+        self.assertEqual(ret, {'a': 1, 'b': 2})
+        self.assertIs(type(ret), dict)
+
+        ret = get_kwargs(a=1, **{'b': 2, 'c': 3})
+        self.assertEqual(ret, {'a': 1, 'b': 2, 'c': 3})
+        self.assertIs(type(ret), dict)
+
+        ret = get_kwargs(**DictSubclass({'a': 1, 'b': 2}))
+        self.assertEqual(ret, {'a': 1, 'b': 2})
+        self.assertIs(type(ret), dict)
+
+        ret = get_kwargs()
+        self.assertIn(ret, ({}, None))
+        self.assertIn(type(ret), (dict, type(None)))
+
+        ret = get_kwargs(**{})
+        self.assertIn(ret, ({}, None))
+        self.assertIn(type(ret), (dict, type(None)))
+
     def test_positional_args(self):
         # using all positional args
         self.assertEqual(
diff --git a/Misc/NEWS b/Misc/NEWS
index 727f84e143f..63cdce28349 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -10,6 +10,9 @@ Release date: tba
 Core and Builtins
 -----------------
 
+- Issue #18531: Single var-keyword argument of dict subtype was passed
+  unscathed to the C-defined function.  Now it is converted to exact dict.
+
 - Issue #26811: gc.get_objects() no longer contains a broken tuple with NULL
   pointer.
 
diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index 8c794859ebe..5083be64023 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -872,6 +872,26 @@ test_L_code(PyObject *self)
 
 #endif  /* ifdef HAVE_LONG_LONG */
 
+static PyObject *
+get_args(PyObject *self, PyObject *args)
+{
+    if (args == NULL) {
+        args = Py_None;
+    }
+    Py_INCREF(args);
+    return args;
+}
+
+static PyObject *
+get_kwargs(PyObject *self, PyObject *args, PyObject *kwargs)
+{
+    if (kwargs == NULL) {
+        kwargs = Py_None;
+    }
+    Py_INCREF(kwargs);
+    return kwargs;
+}
+
 /* Test tuple argument processing */
 static PyObject *
 getargs_tuple(PyObject *self, PyObject *args)
@@ -3784,6 +3804,8 @@ static PyMethodDef TestMethods[] = {
     {"test_pep3118_obsolete_write_locks", (PyCFunction)test_pep3118_obsolete_write_locks, METH_NOARGS},
 #endif
     {"getbuffer_with_null_view", getbuffer_with_null_view, METH_O},
+    {"get_args", get_args, METH_VARARGS},
+    {"get_kwargs", (PyCFunction)get_kwargs, METH_VARARGS|METH_KEYWORDS},
     {"getargs_tuple",           getargs_tuple,                   METH_VARARGS},
     {"getargs_keywords", (PyCFunction)getargs_keywords,
       METH_VARARGS|METH_KEYWORDS},
diff --git a/Python/ceval.c b/Python/ceval.c
index 88dc1139eb0..b4610300602 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -4993,7 +4993,7 @@ ext_do_call(PyObject *func, PyObject ***pp_stack, int flags, int na, int nk)
 
     if (flags & CALL_FLAG_KW) {
         kwdict = EXT_POP(*pp_stack);
-        if (!PyDict_Check(kwdict)) {
+        if (!PyDict_CheckExact(kwdict)) {
             PyObject *d;
             d = PyDict_New();
             if (d == NULL)