diff --git a/include/mbedtls/cmac.h b/include/mbedtls/cmac.h
index b67305c..00aeaa8 100644
--- a/include/mbedtls/cmac.h
+++ b/include/mbedtls/cmac.h
@@ -77,6 +77,12 @@
  *                      the input data.
  *                      Must be called with an initialized cipher context.
  *
+ * \note                When the CMAC implementation is supplied by an alternate
+ *                      implementation (through #MBEDTLS_CMAC_ALT), some ciphers
+ *                      may not be supported by that implementation, and thus
+ *                      return an error. Alternate implementations must support
+ *                      AES-128 and AES-256, and may support AES-192 and 3DES.
+ *
  * \param ctx           The cipher context used for the CMAC operation, initialized
  *                      as one of the following types: MBEDTLS_CIPHER_AES_128_ECB,
  *                      MBEDTLS_CIPHER_AES_192_ECB, MBEDTLS_CIPHER_AES_256_ECB,
@@ -154,6 +160,11 @@
  *                      The CMAC result is calculated as
  *                      output = generic CMAC(cmac key, input buffer).
  *
+ * \note                When the CMAC implementation is supplied by an alternate
+ *                      implementation (through #MBEDTLS_CMAC_ALT), some ciphers
+ *                      may not be supported by that implementation, and thus
+ *                      return an error. Alternate implementations must support
+ *                      AES-128 and AES-256, and may support AES-192 and 3DES.
  *
  * \param cipher_info   The cipher information.
  * \param key           The CMAC key.