diff --git a/docs/x509/reference.rst b/docs/x509/reference.rst
index 87383db..c8c58ef 100644
--- a/docs/x509/reference.rst
+++ b/docs/x509/reference.rst
@@ -503,6 +503,8 @@
              handled by whoever reads the certificate.
 
     .. method:: sign(private_key, algorithm, backend)
+    
+        :type: :class:`~cryptography.x509.Certificate`
 
         Sign the certificate using the CA's private key.