diff --git a/include/mbedtls/aes.h b/include/mbedtls/aes.h
index b5560cc..660ec2a 100644
--- a/include/mbedtls/aes.h
+++ b/include/mbedtls/aes.h
@@ -38,6 +38,7 @@
 
 #define MBEDTLS_ERR_AES_INVALID_KEY_LENGTH                -0x0020  /**< Invalid key length. */
 #define MBEDTLS_ERR_AES_INVALID_INPUT_LENGTH              -0x0022  /**< Invalid data input length. */
+#define MBEDTLS_ERR_AES_FEATURE_UNAVAILABLE               -0x006E  /**< Feature not available, e.g. unsupported AES key size. */
 
 #if ( defined(__ARMCC_VERSION) || defined(_MSC_VER) ) && \
     !defined(inline) && !defined(__cplusplus)
diff --git a/include/mbedtls/error.h b/include/mbedtls/error.h
index 5e549f6..8dfeb62 100644
--- a/include/mbedtls/error.h
+++ b/include/mbedtls/error.h
@@ -52,7 +52,7 @@
  * GCM       2  0x0012-0x0014
  * BLOWFISH  2  0x0016-0x0018
  * THREADING 3  0x001A-0x001E
- * AES       2  0x0020-0x0022
+ * AES       2  0x0020-0x0022   0x006E-0x006E
  * CAMELLIA  2  0x0024-0x0026
  * XTEA      1  0x0028-0x0028
  * BASE64    2  0x002A-0x002C
diff --git a/library/error.c b/library/error.c
index dd2db0c..11f7c60 100644
--- a/library/error.c
+++ b/library/error.c
@@ -516,6 +516,8 @@
         mbedtls_snprintf( buf, buflen, "AES - Invalid key length" );
     if( use_ret == -(MBEDTLS_ERR_AES_INVALID_INPUT_LENGTH) )
         mbedtls_snprintf( buf, buflen, "AES - Invalid data input length" );
+    if( use_ret == -(MBEDTLS_ERR_AES_FEATURE_UNAVAILABLE) )
+        mbedtls_snprintf( buf, buflen, "AES - Feature not available, e.g. unsupported AES key size" );
 #endif /* MBEDTLS_AES_C */
 
 #if defined(MBEDTLS_ASN1_PARSE_C)