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.