diff --git a/Modules/_json.c b/Modules/_json.c
index 6cc31c6e371224..4887210bdb45e0 100644
--- a/Modules/_json.c
+++ b/Modules/_json.c
@@ -1503,6 +1503,14 @@ encoder_listencode_obj(PyEncoderObject *s, _PyAccu *acc,
             }
         }
         newobj = PyObject_CallFunctionObjArgs(s->defaultfn, obj, NULL);
+        if (newobj != NULL && Py_TYPE(newobj) == Py_TYPE(obj)) {
+            PyErr_SetString(PyExc_TypeError, "Default serialization function "
+                            "returning same type");
+            Py_DECREF(newobj);
+            Py_XDECREF(ident);
+            return -1;
+        }
+
         if (newobj == NULL) {
             Py_XDECREF(ident);
             return -1;