diff --git a/doc/system.adoc b/doc/system.adoc
index f135c08..06b067d 100644
--- a/doc/system.adoc
+++ b/doc/system.adoc
@@ -4,7 +4,7 @@ Copyright 2018 Peter Dimov
 Distributed under the Boost Software License, Version 1.0.
 
 See accompanying file LICENSE_1_0.txt or copy at
-http://www.boost.org/LICENSE_1_0.txt
+https://www.boost.org/LICENSE_1_0.txt
 ////
 
 # Boost.System: Extensible Error Reporting