diff --git a/src/cryptography/x509.py b/src/cryptography/x509.py
index 668bc2e..a091cd7 100644
--- a/src/cryptography/x509.py
+++ b/src/cryptography/x509.py
@@ -1472,6 +1472,10 @@
             extension = Extension(
                 OID_SUBJECT_ALTERNATIVE_NAME, critical, extension
             )
+        elif isinstance(extension, KeyUsage):
+            extension = Extension(
+                OID_KEY_USAGE, critical, extension
+            )
         else:
             raise NotImplementedError('Unsupported X.509 extension.')
         # TODO: This is quadratic in the number of extensions
diff --git a/tests/test_x509.py b/tests/test_x509.py
index 64a5923..133f053 100644
--- a/tests/test_x509.py
+++ b/tests/test_x509.py
@@ -864,7 +864,17 @@
                 x509.NameAttribute(x509.OID_COUNTRY_NAME, u'US'),
             ])
         ).add_extension(
-            x509.SubjectAlternativeName([x509.DNSName(u"cryptography.io")]),
+            x509.KeyUsage(
+                digital_signature=True,
+                content_commitment=True,
+                key_encipherment=False,
+                data_encipherment=False,
+                key_agreement=False,
+                key_cert_sign=True,
+                crl_sign=False,
+                encipher_only=False,
+                decipher_only=False
+            ),
             critical=False,
         )
         with pytest.raises(NotImplementedError):