diff --git a/include/mbedtls/ssl_internal.h b/include/mbedtls/ssl_internal.h
index fced2cb..a13feb5 100644
--- a/include/mbedtls/ssl_internal.h
+++ b/include/mbedtls/ssl_internal.h
@@ -765,6 +765,7 @@
 
 #if defined(MBEDTLS_SSL_PROTO_TLS1) || defined(MBEDTLS_SSL_PROTO_TLS1_1) || \
     defined(MBEDTLS_SSL_PROTO_TLS1_2)
+// The hash buffer must have at least MBEDTLS_MD_MAX_SIZE bytes of length.
 int mbedtls_ssl_get_key_exchange_md_tls1_2( mbedtls_ssl_context *ssl,
                                             unsigned char *hash, size_t *hashlen,
                                             unsigned char *data, size_t data_len,