diff --git a/include/boost/beast/http/serializer.hpp b/include/boost/beast/http/serializer.hpp
index f2922608..c12bc172 100644
--- a/include/boost/beast/http/serializer.hpp
+++ b/include/boost/beast/http/serializer.hpp
@@ -286,7 +286,7 @@ public:
         successfully retrieved.
     */
     bool
-    is_done()
+    is_done() const
     {
         return s_ == do_complete;
     }