Document how long iv should be
diff --git a/docs/primitives/symmetric-encryption.rst b/docs/primitives/symmetric-encryption.rst
index 4675ebf..c0b3608 100644
--- a/docs/primitives/symmetric-encryption.rst
+++ b/docs/primitives/symmetric-encryption.rst
@@ -52,4 +52,6 @@
:param bytes initialization_vector: Must be random bytes. They do not need
to be kept secret (they can be included
- in a transmitted message).
+ in a transmitted message). Should be
+ the same number of bytes as the ``key``
+ for the cipher.