diff --git a/Lib/test/test_unicode.py b/Lib/test/test_unicode.py
index 7b130ca..0bc1525 100644
--- a/Lib/test/test_unicode.py
+++ b/Lib/test/test_unicode.py
@@ -11,6 +11,7 @@ import sys
 import unittest
 import warnings
 from test import support, string_tests
+import _string
 
 # Error handling (bad decoder return)
 def search_function(encoding):
@@ -1516,6 +1517,33 @@ class UnicodeTest(string_tests.CommonTest,
         self.assertEqual(wchar, nonbmp + '\0')
 
 
+class StringModuleTest(unittest.TestCase):
+    def test_formatter_parser(self):
+        def parse(format):
+            return list(_string.formatter_parser(format))
+        formatter = parse("prefix {2!s}xxx{0:^+10.3f}{obj.attr!s}")
+        self.assertEqual(formatter, [
+            ('prefix ', '2', '', 's'),
+            ('xxx', '0', '^+10.3f', None),
+            ('', 'obj.attr', '', 's'),
+        ])
+        self.assertRaises(TypeError, _string.formatter_parser, 1)
+
+    def test_formatter_field_name_split(self):
+        def split(name):
+            items = list(_string.formatter_field_name_split(name))
+            items[1] = list(items[1])
+            return items
+        self.assertEqual(split("obj.arg"), ["obj", [(True, 'arg')]])
+        self.assertEqual(split("obj.arg[key1][key2]"), [
+            "obj",
+            [(True, 'arg'),
+             (False, 'key1'),
+             (False, 'key2'),
+            ]])
+        self.assertRaises(TypeError, _string.formatter_field_name_split, 1)
+
+
 def test_main():
     support.run_unittest(__name__)
 
diff --git a/Objects/stringlib/string_format.h b/Objects/stringlib/string_format.h
index 4053545..2c09e26 100644
--- a/Objects/stringlib/string_format.h
+++ b/Objects/stringlib/string_format.h
@@ -1192,6 +1192,11 @@ formatter_parser(PyObject *ignored, STRINGLIB_OBJECT *self)
 {
     formatteriterobject *it;
 
+    if (!PyUnicode_Check(self)) {
+        PyErr_Format(PyExc_TypeError, "expect str, not %s", Py_TYPE(self)->tp_name);
+        return NULL;
+    }
+
     it = PyObject_New(formatteriterobject, &PyFormatterIter_Type);
     if (it == NULL)
         return NULL;
@@ -1332,6 +1337,11 @@ formatter_field_name_split(PyObject *ignored, STRINGLIB_OBJECT *self)
     PyObject *first_obj = NULL;
     PyObject *result = NULL;
 
+    if (!PyUnicode_Check(self)) {
+        PyErr_Format(PyExc_TypeError, "expect str, not %s", Py_TYPE(self)->tp_name);
+        return NULL;
+    }
+
     it = PyObject_New(fieldnameiterobject, &PyFieldNameIter_Type);
     if (it == NULL)
         return NULL;