diff --git a/include/boost/asio/async_result.hpp b/include/boost/asio/async_result.hpp
index e9bd3419..c73e0eed 100644
--- a/include/boost/asio/async_result.hpp
+++ b/include/boost/asio/async_result.hpp
@@ -1321,7 +1321,7 @@ class async_result<detail::async_operation_probe, Sig0, Sig1, Sig2>
 #endif // defined(BOOST_ASIO_HAS_VARIADIC_TEMPLATES)
 #endif // !defined(GENERATING_DOCUMENTATION)
 
-#if defined(GENERATION_DOCUMENTATION)
+#if defined(GENERATING_DOCUMENTATION)
 
 /// The is_async_operation trait detects whether a type @c T and arguments
 /// @c Args... may be used to initiate an asynchronous operation.