diff --git a/ChangeLog.d/fix_tls_alert_codes.txt b/ChangeLog.d/fix_tls_alert_codes.txt
new file mode 100644
index 0000000..abe3a5e
--- /dev/null
+++ b/ChangeLog.d/fix_tls_alert_codes.txt
@@ -0,0 +1,6 @@
+Bugfix
+   * Fix the alert raised when a client requests an invalid
+   * fragment length, as per RFC6066 section 4. We now alert with
+   * MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER. Similarly, raise
+   * MBEDTLS_SSL_ALERT_MSG_DECRYPT_ERROR for an invalid finished
+   * message, as per RFC5247 section 7.2.2.
diff --git a/library/ssl_cli.c b/library/ssl_cli.c
index 01e3f11..6710f13 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -1525,7 +1525,7 @@
         mbedtls_ssl_send_alert_message(
             ssl,
             MBEDTLS_SSL_ALERT_LEVEL_FATAL,
-            MBEDTLS_SSL_ALERT_MSG_HANDSHAKE_FAILURE );
+            MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
         return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
     }
 
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index a4cf44c..976a87c 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -3610,7 +3610,7 @@
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad finished message" ) );
         mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
-                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
+                                        MBEDTLS_SSL_ALERT_MSG_DECRYPT_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_FINISHED );
     }