Be explicit
diff --git a/docs/fernet.rst b/docs/fernet.rst
index ac610eb..d44e737 100644
--- a/docs/fernet.rst
+++ b/docs/fernet.rst
@@ -44,7 +44,9 @@
:param int ttl: Optionally, the number of seconds old a message may be
for it to be valid. If the message is older than
``ttl`` seconds (from the time it was originally
- created) an exception will be raised.
+ created) an exception will be raised. If ``ttl`` is not
+ provided (or is ``None``), the age of the message is
+ not considered.
:returns bytes: The original plaintext.