diff --git a/cert_chain_resolver/models.py b/cert_chain_resolver/models.py
index 2f70915..b28d41c 100644
--- a/cert_chain_resolver/models.py
+++ b/cert_chain_resolver/models.py
@@ -49,9 +49,9 @@ def __repr__(self):
         )
 
     def __eq__(self, other):
-        # type: (object) -> bool
+        # type: (Cert) -> bool
         if not isinstance(other, Cert):
-            return NotImplemented
+            raise TypeError
         return self.fingerprint == other.fingerprint
 
     @property
diff --git a/tests/test_cli.py b/tests/test_cli.py
index 12c1a87..e40ee74 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -1,3 +1,5 @@
+import importlib
+import sys
 from tempfile import NamedTemporaryFile
 import pytest
 from cert_chain_resolver import __is_py3__
@@ -222,3 +224,12 @@ def test_main_handles_different_file_input(mocker, file_name, expected_content):
         include_root=False,
         root_ca_store=mocker.ANY,
     )
+
+
+def test_main_no_args_tty_shows_help_and_exits(mocker):
+    mocker.patch("sys.stdin.isatty", return_value=True)
+    mocker.patch("sys.argv", ["script_name"])
+
+    with pytest.raises(SystemExit):
+        main()
+    assert sys.argv == ["script_name", "-h"]
diff --git a/tests/test_models.py b/tests/test_models.py
index 7fd38c0..abf4aca 100644
--- a/tests/test_models.py
+++ b/tests/test_models.py
@@ -1,4 +1,5 @@
 from cert_chain_resolver.exceptions import MissingCertProperty
+from contextlib import nullcontext as does_not_raise
 from .fixtures import BUNDLE_FIXTURES, CERT_FIXTURES, certfixture_to_id
 from cryptography import x509
 from cert_chain_resolver.models import Cert, CertificateChain
@@ -29,6 +30,16 @@ def test_certcontainer_x509_helper_props(cert):
     assert fixture["ca_issuer_access_location"] == c.ca_issuer_access_location
 
 
+def test_cert_constructor_requires_x509():
+    with pytest.raises(TypeError, match="Argument must be a x509"):
+        Cert("not a x509 obj")
+
+
+def test_cert__eq__raises(mocker):
+    with pytest.raises(TypeError):
+        Cert(mocker.Mock(spec=x509.Certificate)).__eq__("Not a Cert")
+
+
 @pytest.mark.parametrize(
     "_subject",
     ["CA - XD 9001", pytest.param("CN=github.com,O=GitHub", marks=[pytest.mark.xfail])],
@@ -137,6 +148,28 @@ def test_missing_cert_properties_raise(mocker, prop, cert_prop, cert_value):
         getattr(c, prop)
 
 
+@pytest.mark.parametrize(
+    "value,expectation",
+    [
+        (b"Common name", does_not_raise()),
+        (unicode("Common name"), does_not_raise()),
+        (["unexpected type"], pytest.raises(ValueError)),
+    ],
+)
+def test_common_name_handles_unicode_or_bytes(mocker, value, expectation):
+    m = mocker.Mock(
+        spec=x509.Certificate,
+        subject=mocker.Mock(
+            get_attributes_for_oid=mocker.Mock(
+                return_value=[mocker.Mock(spec=type(value), value=value)]
+            )
+        ),
+    )
+    with expectation:
+        c = Cert(m)
+        assert c.common_name == "Common name"
+
+
 def test_repr():
     class CertOverride(Cert):
         subject = "Subject"