diff --git a/cryptography/hazmat/backends/openssl/backend.py b/cryptography/hazmat/backends/openssl/backend.py
index 0692492..4137ad1 100644
--- a/cryptography/hazmat/backends/openssl/backend.py
+++ b/cryptography/hazmat/backends/openssl/backend.py
@@ -1254,7 +1254,6 @@
 @utils.register_interface(interfaces.CMACContext)
 class _CMACContext(object):
     def __init__(self, backend, algorithm, ctx=None):
-
         if not backend.cmac_algorithm_supported(algorithm):
             raise UnsupportedAlgorithm("This backend does not support CMAC")
 
@@ -1317,7 +1316,7 @@
         res = self._backend._lib.CMAC_CTX_copy(
             copied_ctx, self._ctx
         )
-        assert res != 0
+        assert res == 0
         return _CMACContext(
             self._backend, self.algorithm, ctx=copied_ctx
         )
diff --git a/docs/hazmat/primitives/mac/cmac.rst b/docs/hazmat/primitives/mac/cmac.rst
index be35014..8b88a3c 100644
--- a/docs/hazmat/primitives/mac/cmac.rst
+++ b/docs/hazmat/primitives/mac/cmac.rst
@@ -15,6 +15,8 @@
 secret key. You can use an CMAC to verify both the integrity and authenticity
 of a message.
 
+A subset of CMAC with the AES-128 algorithm is described in :rfc:`4493`.
+
 .. class:: CMAC(algorithm, backend)
 
     CMAC objects take a