diff --git a/src/json.hpp b/src/json.hpp
index e20ab71d41..efd76da7d0 100644
--- a/src/json.hpp
+++ b/src/json.hpp
@@ -14492,7 +14492,8 @@ struct hash<nlohmann::json>
 
 /// specialization for std::less<value_t>
 template<>
-struct less<::nlohmann::detail::value_t>
+struct less< ::nlohmann::detail::value_t> // do not remove the space after '<', 
+                                          // see https://github.com/nlohmann/json/pull/679
 {
     /*!
     @brief compare two value_t enum values