diff --git a/include/boost/asio/detail/posix_fd_set_adapter.hpp b/include/boost/asio/detail/posix_fd_set_adapter.hpp
index 6a3ce08e..b4a26091 100644
--- a/include/boost/asio/detail/posix_fd_set_adapter.hpp
+++ b/include/boost/asio/detail/posix_fd_set_adapter.hpp
@@ -59,7 +59,7 @@ public:
   }
 
 private:
-  fd_set fd_set_;
+  mutable fd_set fd_set_;
   socket_type max_descriptor_;
 };