diff --git a/include/mbedtls/aes.h b/include/mbedtls/aes.h
index c53f817..c43134d 100644
--- a/include/mbedtls/aes.h
+++ b/include/mbedtls/aes.h
@@ -60,6 +60,8 @@
 /* Error codes in range 0x0021-0x0025 */
 /** Invalid input data. */
 #define MBEDTLS_ERR_AES_BAD_INPUT_DATA                    -0x0021
+/** The requested feature is not available. */
+#define MBEDTLS_ERR_AES_FEATURE_UNAVAILABLE               -0x0023
 
 #ifdef __cplusplus
 extern "C" {