diff --git a/include/mbedtls/des.h b/include/mbedtls/des.h
index 91d16b6..d62042d 100644
--- a/include/mbedtls/des.h
+++ b/include/mbedtls/des.h
@@ -42,6 +42,8 @@
 #define MBEDTLS_DES_DECRYPT     0
 
 #define MBEDTLS_ERR_DES_INVALID_INPUT_LENGTH              -0x0032  /**< The data input has an invalid length. */
+
+/* MBEDTLS_ERR_DES_HW_ACCEL_FAILED is deprecated and should not be used. */
 #define MBEDTLS_ERR_DES_HW_ACCEL_FAILED                   -0x0033  /**< DES hardware accelerator failed. */
 
 #define MBEDTLS_DES_KEY_SIZE    8