diff --git a/Lib/test/test_format.py b/Lib/test/test_format.py
index e9e5bb9..dff8c69 100644
--- a/Lib/test/test_format.py
+++ b/Lib/test/test_format.py
@@ -1,6 +1,7 @@
 from test.support import verbose, TestFailed
 import locale
 import sys
+import re
 import test.support as support
 import unittest
 
@@ -495,6 +496,25 @@
         self.assertEqual(format(12300050.0, ".6g"), "1.23e+07")
         self.assertEqual(format(12300050.0, "#.6g"), "1.23000e+07")
 
+    def test_with_two_commas_in_format_specifier(self):
+        error_msg = re.escape("Cannot specify ',' with ','.")
+        with self.assertRaisesRegex(ValueError, error_msg):
+            '{:,,}'.format(1)
+
+    def test_with_two_underscore_in_format_specifier(self):
+        error_msg = re.escape("Cannot specify '_' with '_'.")
+        with self.assertRaisesRegex(ValueError, error_msg):
+            '{:__}'.format(1)
+
+    def test_with_a_commas_and_an_underscore_in_format_specifier(self):
+        error_msg = re.escape("Cannot specify both ',' and '_'.")
+        with self.assertRaisesRegex(ValueError, error_msg):
+            '{:,_}'.format(1)
+
+    def test_with_an_underscore_and_a_comma_in_format_specifier(self):
+        error_msg = re.escape("Cannot specify both ',' and '_'.")
+        with self.assertRaisesRegex(ValueError, error_msg):
+            '{:,_}'.format(1)
 
 if __name__ == "__main__":
     unittest.main()
diff --git a/Lib/test/test_fstring.py b/Lib/test/test_fstring.py
index 35a62a0..b9bede0 100644
--- a/Lib/test/test_fstring.py
+++ b/Lib/test/test_fstring.py
@@ -9,6 +9,7 @@
 
 import ast
 import os
+import re
 import types
 import decimal
 import unittest
@@ -1198,6 +1199,25 @@
         with self.assertRaisesRegex(SyntaxError, "f-string: invalid syntax"):
             compile("f'{a $ b}'", "?", "exec")
 
+    def test_with_two_commas_in_format_specifier(self):
+        error_msg = re.escape("Cannot specify ',' with ','.")
+        with self.assertRaisesRegex(ValueError, error_msg):
+            f'{1:,,}'
+
+    def test_with_two_underscore_in_format_specifier(self):
+        error_msg = re.escape("Cannot specify '_' with '_'.")
+        with self.assertRaisesRegex(ValueError, error_msg):
+            f'{1:__}'
+
+    def test_with_a_commas_and_an_underscore_in_format_specifier(self):
+        error_msg = re.escape("Cannot specify both ',' and '_'.")
+        with self.assertRaisesRegex(ValueError, error_msg):
+            f'{1:,_}'
+
+    def test_with_an_underscore_and_a_comma_in_format_specifier(self):
+        error_msg = re.escape("Cannot specify both ',' and '_'.")
+        with self.assertRaisesRegex(ValueError, error_msg):
+            f'{1:,_}'
 
 if __name__ == '__main__':
     unittest.main()