diff --git a/tests/test_x509.py b/tests/test_x509.py
index ee83ed2..99551eb 100644
--- a/tests/test_x509.py
+++ b/tests/test_x509.py
@@ -834,7 +834,7 @@
         assert basic_constraints.value.ca is True
         assert basic_constraints.value.path_length == 2
 
-    def test_add_duplicate_extension(self, backend):
+    def test_add_duplicate_extension(self):
         builder = x509.CertificateSigningRequestBuilder().add_extension(
             x509.BasicConstraints(True, 2), critical=True,
         )
@@ -843,12 +843,12 @@
                 x509.BasicConstraints(True, 2), critical=True,
             )
 
-    def test_set_invalid_subject(self, backend):
+    def test_set_invalid_subject(self):
         builder = x509.CertificateSigningRequestBuilder()
         with pytest.raises(TypeError):
             builder.subject_name('NotAName')
 
-    def test_add_unsupported_extension(self, backend):
+    def test_add_unsupported_extension(self):
         builder = x509.CertificateSigningRequestBuilder()
         with pytest.raises(NotImplementedError):
             builder.add_extension(
@@ -856,6 +856,20 @@
                 critical=False,
             )
 
+    def test_set_subject_twice(self):
+        builder = x509.CertificateSigningRequestBuilder()
+        builder = builder.subject_name(
+            x509.Name([
+                x509.NameAttribute(x509.OID_COUNTRY_NAME, u'US'),
+            ])
+        )
+        with pytest.raises(ValueError):
+            builder.subject_name(
+                x509.Name([
+                    x509.NameAttribute(x509.OID_COUNTRY_NAME, u'US'),
+                ])
+            )
+
 
 @pytest.mark.requires_backend_interface(interface=DSABackend)
 @pytest.mark.requires_backend_interface(interface=X509Backend)