diff --git a/src/OpenSSL/crypto.py b/src/OpenSSL/crypto.py
index 91cc8c8..4a379e8 100644
--- a/src/OpenSSL/crypto.py
+++ b/src/OpenSSL/crypto.py
@@ -1117,9 +1117,7 @@
                 _raise_current_error()
             asn1_serial = _ffi.gc(asn1_serial, _lib.ASN1_INTEGER_free)
             set_result = _lib.X509_set_serialNumber(self._x509, asn1_serial)
-            if not set_result:
-                # TODO Not tested
-                _raise_current_error()
+            _openssl_assert(set_result == 1)
 
     def get_serial_number(self):
         """
diff --git a/tests/test_crypto.py b/tests/test_crypto.py
index 10d2427..81e0ae3 100644
--- a/tests/test_crypto.py
+++ b/tests/test_crypto.py
@@ -959,17 +959,22 @@
         """
         name = self._x509name()
         name.commonName = "foo"
-        self.assertEqual(name.commonName, "foo")
-        self.assertEqual(name.CN, "foo")
+        assert name.commonName == "foo"
+        assert name.CN == "foo"
+
         name.CN = "baz"
-        self.assertEqual(name.commonName, "baz")
-        self.assertEqual(name.CN, "baz")
+        assert name.commonName == "baz"
+        assert name.CN == "baz"
+
         name.commonName = "bar"
-        self.assertEqual(name.commonName, "bar")
-        self.assertEqual(name.CN, "bar")
+        assert name.commonName == "bar"
+        assert name.CN == "bar"
+
         name.CN = "quux"
-        self.assertEqual(name.commonName, "quux")
-        self.assertEqual(name.CN, "quux")
+        assert name.commonName == "quux"
+        assert name.CN == "quux"
+
+        assert name.OU is None
 
         with pytest.raises(AttributeError):
             name.foobar
@@ -1922,6 +1927,15 @@
         cert = load_certificate(FILETYPE_PEM, certPEM)
         self.assertRaises(ValueError, cert.get_signature_algorithm)
 
+    def test_sign_bad_pubkey_type(self):
+        """
+        :obj:`X509.sign` raises :obj:`TypeError` when called with the wrong
+        type.
+        """
+        cert = X509()
+        with pytest.raises(TypeError):
+            cert.sign(object(), b"sha256")
+
 
 class X509StoreTests(TestCase):
     """
@@ -2695,6 +2709,15 @@
         good_text = _runopenssl(dumped_pem, b"x509", b"-noout", b"-text")
         self.assertEqual(dumped_text, good_text)
 
+    def test_dump_certificate_bad_type(self):
+        """
+        :obj:`dump_certificate` raises a :obj:`ValueError` if it's called with
+        a bad type.
+        """
+        cert = load_certificate(FILETYPE_PEM, cleartextCertificatePEM)
+        with pytest.raises(ValueError):
+            dump_certificate(object(), cert)
+
     def test_dump_privatekey_pem(self):
         """
         :py:obj:`dump_privatekey` writes a PEM
@@ -2869,6 +2892,14 @@
         with pytest.raises(ValueError):
             load_certificate(object(), b"")
 
+    def test_bad_certificate(self):
+        """
+        If the bytes passed to :obj:`load_certificate` are not a valid
+        certificate, an exception is raised.
+        """
+        with pytest.raises(Error):
+            load_certificate(FILETYPE_ASN1, b"lol")
+
 
 class PKCS7Tests(TestCase):
     """