diff --git a/Lib/test/test_capi.py b/Lib/test/test_capi.py
index a95e4c5..eb3e2c5 100644
--- a/Lib/test/test_capi.py
+++ b/Lib/test/test_capi.py
@@ -490,9 +490,8 @@
             # test the format unit when not skipped
             format = c + "i"
             try:
-                # (note: the format string must be bytes!)
                 _testcapi.parse_tuple_and_keywords(tuple_1, dict_b,
-                    format.encode("ascii"), keywords)
+                    format, keywords)
                 when_not_skipped = False
             except SystemError as e:
                 s = "argument 1 (impossible<bad format char>)"
@@ -504,7 +503,7 @@
             optional_format = "|" + format
             try:
                 _testcapi.parse_tuple_and_keywords(empty_tuple, dict_b,
-                    optional_format.encode("ascii"), keywords)
+                    optional_format, keywords)
                 when_skipped = False
             except SystemError as e:
                 s = "impossible<bad format char>: '{}'".format(format)
@@ -517,40 +516,64 @@
             self.assertIs(when_skipped, when_not_skipped, message)
 
     def test_parse_tuple_and_keywords(self):
-        # parse_tuple_and_keywords error handling tests
+        # Test handling errors in the parse_tuple_and_keywords helper itself
         self.assertRaises(TypeError, _testcapi.parse_tuple_and_keywords,
                           (), {}, 42, [])
         self.assertRaises(ValueError, _testcapi.parse_tuple_and_keywords,
-                          (), {}, b'', 42)
+                          (), {}, '', 42)
         self.assertRaises(ValueError, _testcapi.parse_tuple_and_keywords,
-                          (), {}, b'', [''] * 42)
+                          (), {}, '', [''] * 42)
         self.assertRaises(ValueError, _testcapi.parse_tuple_and_keywords,
-                          (), {}, b'', [42])
+                          (), {}, '', [42])
+
+    def test_bad_use(self):
+        # Test handling invalid format and keywords in
+        # PyArg_ParseTupleAndKeywords()
+        self.assertRaises(SystemError, _testcapi.parse_tuple_and_keywords,
+                          (1,), {}, '||O', ['a'])
+        self.assertRaises(SystemError, _testcapi.parse_tuple_and_keywords,
+                          (1, 2), {}, '|O|O', ['a', 'b'])
+        self.assertRaises(SystemError, _testcapi.parse_tuple_and_keywords,
+                          (), {'a': 1}, '$$O', ['a'])
+        self.assertRaises(SystemError, _testcapi.parse_tuple_and_keywords,
+                          (), {'a': 1, 'b': 2}, '$O$O', ['a', 'b'])
+        self.assertRaises(SystemError, _testcapi.parse_tuple_and_keywords,
+                          (), {'a': 1}, '$|O', ['a'])
+        self.assertRaises(SystemError, _testcapi.parse_tuple_and_keywords,
+                          (), {'a': 1, 'b': 2}, '$O|O', ['a', 'b'])
+        self.assertRaises(SystemError, _testcapi.parse_tuple_and_keywords,
+                          (1,), {}, '|O', ['a', 'b'])
+        self.assertRaises(SystemError, _testcapi.parse_tuple_and_keywords,
+                          (1,), {}, '|OO', ['a'])
+        self.assertRaises(SystemError, _testcapi.parse_tuple_and_keywords,
+                          (), {}, '|$O', [''])
+        self.assertRaises(SystemError, _testcapi.parse_tuple_and_keywords,
+                          (), {}, '|OO', ['a', ''])
 
     def test_positional_only(self):
         parse = _testcapi.parse_tuple_and_keywords
 
-        parse((1, 2, 3), {}, b'OOO', ['', '', 'a'])
-        parse((1, 2), {'a': 3}, b'OOO', ['', '', 'a'])
+        parse((1, 2, 3), {}, 'OOO', ['', '', 'a'])
+        parse((1, 2), {'a': 3}, 'OOO', ['', '', 'a'])
         with self.assertRaisesRegex(TypeError,
                r'function takes at least 2 positional arguments \(1 given\)'):
-            parse((1,), {'a': 3}, b'OOO', ['', '', 'a'])
-        parse((1,), {}, b'O|OO', ['', '', 'a'])
+            parse((1,), {'a': 3}, 'OOO', ['', '', 'a'])
+        parse((1,), {}, 'O|OO', ['', '', 'a'])
         with self.assertRaisesRegex(TypeError,
                r'function takes at least 1 positional arguments \(0 given\)'):
-            parse((), {}, b'O|OO', ['', '', 'a'])
-        parse((1, 2), {'a': 3}, b'OO$O', ['', '', 'a'])
+            parse((), {}, 'O|OO', ['', '', 'a'])
+        parse((1, 2), {'a': 3}, 'OO$O', ['', '', 'a'])
         with self.assertRaisesRegex(TypeError,
                r'function takes exactly 2 positional arguments \(1 given\)'):
-            parse((1,), {'a': 3}, b'OO$O', ['', '', 'a'])
-        parse((1,), {}, b'O|O$O', ['', '', 'a'])
+            parse((1,), {'a': 3}, 'OO$O', ['', '', 'a'])
+        parse((1,), {}, 'O|O$O', ['', '', 'a'])
         with self.assertRaisesRegex(TypeError,
                r'function takes at least 1 positional arguments \(0 given\)'):
-            parse((), {}, b'O|O$O', ['', '', 'a'])
+            parse((), {}, 'O|O$O', ['', '', 'a'])
         with self.assertRaisesRegex(SystemError, r'Empty parameter name after \$'):
-            parse((1,), {}, b'O|$OO', ['', '', 'a'])
+            parse((1,), {}, 'O|$OO', ['', '', 'a'])
         with self.assertRaisesRegex(SystemError, 'Empty keyword'):
-            parse((1,), {}, b'O|OO', ['', 'a', ''])
+            parse((1,), {}, 'O|OO', ['', 'a', ''])
 
 
 @unittest.skipUnless(threading, 'Threading required for this test.')
diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index 5460587..f9de940 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -1560,7 +1560,7 @@
 {
     PyObject *sub_args;
     PyObject *sub_kwargs;
-    char *sub_format;
+    const char *sub_format;
     PyObject *sub_keywords;
 
     Py_ssize_t i, size;
@@ -1573,7 +1573,7 @@
 
     double buffers[8][4]; /* double ensures alignment where necessary */
 
-    if (!PyArg_ParseTuple(args, "OOyO:parse_tuple_and_keywords",
+    if (!PyArg_ParseTuple(args, "OOsO:parse_tuple_and_keywords",
         &sub_args, &sub_kwargs,
         &sub_format, &sub_keywords))
         return NULL;