diff --git a/tests/hazmat/backends/test_openssl.py b/tests/hazmat/backends/test_openssl.py
index 1c7a718..73aaa96 100644
--- a/tests/hazmat/backends/test_openssl.py
+++ b/tests/hazmat/backends/test_openssl.py
@@ -50,11 +50,6 @@
             skip_if_libre_ssl(u"LibreSSL 2.1.6")
 
 
-@utils.register_interface(x509.ExtensionType)
-class DummyExtension(object):
-    oid = x509.ObjectIdentifier("1.2.3.4")
-
-
 @utils.register_interface(Mode)
 class DummyMode(object):
     name = "dummy-mode"
@@ -519,27 +514,6 @@
         with pytest.raises(TypeError):
             backend.create_x509_certificate(object(), private_key, DummyHash())
 
-    def test_checks_for_unsupported_extensions(self):
-        private_key = RSA_KEY_2048.private_key(backend)
-        builder = x509.CertificateBuilder().subject_name(x509.Name([
-            x509.NameAttribute(x509.OID_COUNTRY_NAME, u'US'),
-        ])).issuer_name(x509.Name([
-            x509.NameAttribute(x509.OID_COUNTRY_NAME, u'US'),
-        ])).public_key(
-            private_key.public_key()
-        ).serial_number(
-            777
-        ).not_valid_before(
-            datetime.datetime(1999, 1, 1)
-        ).not_valid_after(
-            datetime.datetime(2020, 1, 1)
-        ).add_extension(
-            DummyExtension(), False
-        )
-
-        with pytest.raises(NotImplementedError):
-            builder.sign(private_key, hashes.SHA1(), backend)
-
 
 class TestOpenSSLSerialisationWithOpenSSL(object):
     def test_pem_password_cb_buffer_too_small(self):
diff --git a/tests/test_x509.py b/tests/test_x509.py
index 9100b44..ce52ffa 100644
--- a/tests/test_x509.py
+++ b/tests/test_x509.py
@@ -835,6 +835,29 @@
 class TestCertificateBuilder(object):
     @pytest.mark.requires_backend_interface(interface=RSABackend)
     @pytest.mark.requires_backend_interface(interface=X509Backend)
+    def test_checks_for_unsupported_extensions(self, backend):
+        private_key = RSA_KEY_2048.private_key(backend)
+        builder = x509.CertificateBuilder().subject_name(x509.Name([
+            x509.NameAttribute(x509.OID_COUNTRY_NAME, u'US'),
+        ])).issuer_name(x509.Name([
+            x509.NameAttribute(x509.OID_COUNTRY_NAME, u'US'),
+        ])).public_key(
+            private_key.public_key()
+        ).serial_number(
+            777
+        ).not_valid_before(
+            datetime.datetime(1999, 1, 1)
+        ).not_valid_after(
+            datetime.datetime(2020, 1, 1)
+        ).add_extension(
+            DummyExtension(), False
+        )
+
+        with pytest.raises(NotImplementedError):
+            builder.sign(private_key, hashes.SHA1(), backend)
+
+    @pytest.mark.requires_backend_interface(interface=RSABackend)
+    @pytest.mark.requires_backend_interface(interface=X509Backend)
     def test_no_subject_name(self, backend):
         subject_private_key = RSA_KEY_2048.private_key(backend)
         builder = x509.CertificateBuilder().serial_number(