diff --git a/keyring/cli.py b/keyring/cli.py
index 2c0ba4d3..4ea346de 100644
--- a/keyring/cli.py
+++ b/keyring/cli.py
@@ -91,9 +91,15 @@ def __init__(self):
             'service',
             nargs="?",
         )
+        try:
+            default_user = getpass.getuser()
+        except (OSError, KeyError, ModuleNotFoundError):
+            # on recent Python versions (>=3.13), `OSError` should suffice
+            default_user = None
         self.parser.add_argument(
             'username',
             nargs="?",
+            default=default_user,
         )
         completion.install(self.parser)
 
diff --git a/pyproject.toml b/pyproject.toml
index 18bb6eb0..f1e431fc 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -79,6 +79,9 @@ type = [
 	# upstream
 	"pytest-mypy >= 1.0.1",
 
+	## workaround for python/mypy#20454
+	"mypy < 1.19; python_implementation == 'PyPy'",
+
 	# local
 	"pygobject-stubs",
 	"shtab", # Optional install for completion
diff --git a/tests/test_cli.py b/tests/test_cli.py
index a4bc49c7..15287c49 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -1,5 +1,6 @@
 import getpass
 import itertools
+import os
 import sys
 from unittest import mock
 
@@ -42,6 +43,19 @@ def mocked_get_credential():
         yield get_credential
 
 
+def test_set_no_user(monkeypatch, mocked_set):
+    for name in ('LOGNAME', 'USER', 'LNAME', 'USERNAME'):
+        monkeypatch.delitem(os.environ, name, raising=False)
+    monkeypatch.setattr(os, 'getuid', lambda: -1, raising=False)
+    tool = cli.CommandLineTool()
+    tool.service = 'svc'
+    tool.username = 'usr'
+    monkeypatch.setattr(sys.stdin, 'isatty', lambda: True)
+    monkeypatch.setattr(getpass, 'getpass', PasswordEmitter('foo123'))
+    tool.do_set()
+    mocked_set.assert_called_once_with('svc', 'usr', 'foo123')
+
+
 def test_set_interactive(monkeypatch, mocked_set):
     tool = cli.CommandLineTool()
     tool.service = 'svc'
diff --git a/tox.ini b/tox.ini
index 14243051..e05a3d4a 100644
--- a/tox.ini
+++ b/tox.ini
@@ -20,7 +20,7 @@ deps =
 	diff-cover
 commands =
 	pytest {posargs} --cov-report xml
-	diff-cover coverage.xml --compare-branch=origin/main --html-report diffcov.html
+	diff-cover coverage.xml --compare-branch=origin/main --format html:diffcov.html
 	diff-cover coverage.xml --compare-branch=origin/main --fail-under=100
 
 [testenv:docs]