diff --git a/Lib/argparse.py b/Lib/argparse.py
index c47aeff..8d1a00a 100644
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -153,6 +153,7 @@ def _copy_items(items):
 # Formatting Help
 # ===============
 
+
 class HelpFormatter(object):
     """Formatter for generating usage messages and argument help strings.
 
@@ -695,8 +696,19 @@ class ArgumentDefaultsHelpFormatter(HelpFormatter):
     """
 
     def _get_help_string(self, action):
+        """
+        Add the default value to the option help message.
+
+        ArgumentDefaultsHelpFormatter and BooleanOptionalAction when it isn't
+        already present. This code will do that, detecting cornercases to
+        prevent duplicates or cases where it wouldn't make sense to the end
+        user.
+        """
         help = action.help
-        if '%(default)' not in action.help:
+        if help is None:
+            help = ''
+
+        if '%(default)' not in help:
             if action.default is not SUPPRESS:
                 defaulting_nargs = [OPTIONAL, ZERO_OR_MORE]
                 if action.option_strings or action.nargs in defaulting_nargs:
@@ -704,6 +716,7 @@ def _get_help_string(self, action):
         return help
 
 
+
 class MetavarTypeHelpFormatter(HelpFormatter):
     """Help message formatter which uses the argument 'type' as the default
     metavar value (instead of the argument 'dest')
@@ -719,7 +732,6 @@ def _get_default_metavar_for_positional(self, action):
         return action.type.__name__
 
 
-
 # =====================
 # Options and Arguments
 # =====================
@@ -882,9 +894,6 @@ def __init__(self,
                 option_string = '--no-' + option_string[2:]
                 _option_strings.append(option_string)
 
-        if help is not None and default is not None and default is not SUPPRESS:
-            help += " (default: %(default)s)"
-
         super().__init__(
             option_strings=_option_strings,
             dest=dest,
@@ -896,6 +905,7 @@ def __init__(self,
             help=help,
             metavar=metavar)
 
+
     def __call__(self, parser, namespace, values, option_string=None):
         if option_string in self.option_strings:
             setattr(namespace, self.dest, not option_string.startswith('--no-'))
diff --git a/Lib/test/test_argparse.py b/Lib/test/test_argparse.py
index 794b9df..8509deb 100644
--- a/Lib/test/test_argparse.py
+++ b/Lib/test/test_argparse.py
@@ -3348,6 +3348,7 @@ def _get_parser(self, tester):
             def _test(self, tester, parser_text):
                 expected_text = getattr(tester, self.func_suffix)
                 expected_text = textwrap.dedent(expected_text)
+                tester.maxDiff = None
                 tester.assertEqual(expected_text, parser_text)
 
             def test_format(self, tester):
@@ -3743,7 +3744,7 @@ class TestHelpUsage(HelpTestCase):
           -w W [W ...]          w
           -x [X ...]            x
           --foo, --no-foo       Whether to foo
-          --bar, --no-bar       Whether to bar (default: True)
+          --bar, --no-bar       Whether to bar
           -f, --foobar, --no-foobar, --barfoo, --no-barfoo
           --bazz, --no-bazz     Bazz!
 
@@ -4423,6 +4424,8 @@ class TestHelpArgumentDefaults(HelpTestCase):
         Sig('--bar', action='store_true', help='bar help'),
         Sig('--taz', action=argparse.BooleanOptionalAction,
             help='Whether to taz it', default=True),
+        Sig('--corge', action=argparse.BooleanOptionalAction,
+            help='Whether to corge it', default=argparse.SUPPRESS),
         Sig('--quux', help="Set the quux", default=42),
         Sig('spam', help='spam help'),
         Sig('badger', nargs='?', default='wooden', help='badger help'),
@@ -4432,8 +4435,8 @@ class TestHelpArgumentDefaults(HelpTestCase):
          [Sig('--baz', type=int, default=42, help='baz help')]),
     ]
     usage = '''\
-        usage: PROG [-h] [--foo FOO] [--bar] [--taz | --no-taz] [--quux QUUX]
-                    [--baz BAZ]
+        usage: PROG [-h] [--foo FOO] [--bar] [--taz | --no-taz] [--corge | --no-corge]
+                    [--quux QUUX] [--baz BAZ]
                     spam [badger]
         '''
     help = usage + '''\
@@ -4441,20 +4444,21 @@ class TestHelpArgumentDefaults(HelpTestCase):
         description
 
         positional arguments:
-          spam             spam help
-          badger           badger help (default: wooden)
+          spam                 spam help
+          badger               badger help (default: wooden)
 
         options:
-          -h, --help       show this help message and exit
-          --foo FOO        foo help - oh and by the way, None
-          --bar            bar help (default: False)
-          --taz, --no-taz  Whether to taz it (default: True)
-          --quux QUUX      Set the quux (default: 42)
+          -h, --help           show this help message and exit
+          --foo FOO            foo help - oh and by the way, None
+          --bar                bar help (default: False)
+          --taz, --no-taz      Whether to taz it (default: True)
+          --corge, --no-corge  Whether to corge it
+          --quux QUUX          Set the quux (default: 42)
 
         title:
           description
 
-          --baz BAZ        baz help (default: 42)
+          --baz BAZ            baz help (default: 42)
         '''
     version = ''
 
diff --git a/Misc/NEWS.d/next/Library/2021-08-17-21-41-39.bpo-44587.57OKSz.rst b/Misc/NEWS.d/next/Library/2021-08-17-21-41-39.bpo-44587.57OKSz.rst
new file mode 100644
index 0000000..ce9b3a0
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2021-08-17-21-41-39.bpo-44587.57OKSz.rst
@@ -0,0 +1,2 @@
+Fix BooleanOptionalAction to not automatically add a default string.  If a
+default string is desired, use a formatter to add it.