diff --git a/src/cryptography/hazmat/backends/openssl/backend.py b/src/cryptography/hazmat/backends/openssl/backend.py
index 80b73f4..69a8d87 100644
--- a/src/cryptography/hazmat/backends/openssl/backend.py
+++ b/src/cryptography/hazmat/backends/openssl/backend.py
@@ -1047,7 +1047,7 @@
             elif isinstance(extension.value, x509.SubjectAlternativeName):
                 pp, r = _encode_subject_alt_name(self, extension.value)
             else:
-                raise ValueError('Extension not yet supported.')
+                raise NotImplementedError('Extension not yet supported.')
 
             obj = _txt2obj(self, extension.oid.dotted_string)
             extension = self._lib.X509_EXTENSION_create_by_OBJ(