diff --git a/src/OpenSSL/crypto.py b/src/OpenSSL/crypto.py
index ea7b354..51063b5 100644
--- a/src/OpenSSL/crypto.py
+++ b/src/OpenSSL/crypto.py
@@ -1128,7 +1128,8 @@
 
     def set_version(self, version):
         """
-        Set the version number of the certificate.
+        Set the version number of the certificate. Note that the
+        version value is zero-based, eg. a value of 0 is V1.
 
         :param version: The version number of the certificate.
         :type version: :py:class:`int`