diff --git a/doc/asio_http2.h.rst.in b/doc/asio_http2.h.rst.in
index 6f677440..645ed486 100644
--- a/doc/asio_http2.h.rst.in
+++ b/doc/asio_http2.h.rst.in
@@ -2,3 +2,4 @@ asio_http2.h
 ============
 
 .. literalinclude:: @top_srcdir@/src/includes/nghttp2/asio_http2.h
+   :language: cpp