diff --git a/Lib/test/test_scope.py b/Lib/test/test_scope.py
index 6e46dfa96a6..24a366efc6c 100644
--- a/Lib/test/test_scope.py
+++ b/Lib/test/test_scope.py
@@ -810,6 +810,30 @@ class ScopeTests(unittest.TestCase):
         gc_collect()  # For PyPy or other GCs.
         self.assertIsNone(ref())
 
+    def test_multiple_nesting(self):
+        # Regression test for https://github.com/python/cpython/issues/121863
+        class MultiplyNested:
+            def f1(self):
+                __arg = 1
+                class D:
+                    def g(self, __arg):
+                        return __arg
+                return D().g(_MultiplyNested__arg=2)
+
+            def f2(self):
+                __arg = 1
+                class D:
+                    def g(self, __arg):
+                        return __arg
+                return D().g
+
+        inst = MultiplyNested()
+        with self.assertRaises(TypeError):
+            inst.f1()
+
+        closure = inst.f2()
+        with self.assertRaises(TypeError):
+            closure(_MultiplyNested__arg=2)
 
 if __name__ == '__main__':
     unittest.main()
diff --git a/Objects/codeobject.c b/Objects/codeobject.c
index 7b1244a8d5f..fbc1439d30c 100644
--- a/Objects/codeobject.c
+++ b/Objects/codeobject.c
@@ -147,7 +147,7 @@ intern_strings(PyObject *tuple)
                             "non-string found in code slot");
             return -1;
         }
-        _PyUnicode_InternMortal(interp, &_PyTuple_ITEMS(tuple)[i]);
+        _PyUnicode_InternImmortal(interp, &_PyTuple_ITEMS(tuple)[i]);
     }
     return 0;
 }