diff --git a/beast/http/message.h b/beast/http/message.h
index 1cc21254f..155b8ee36 100644
--- a/beast/http/message.h
+++ b/beast/http/message.h
@@ -30,6 +30,7 @@
 #include <cctype>
 #include <ostream>
 #include <string>
+#include <sstream>
 #include <utility>
 
 namespace beast {