diff --git a/src/json.hpp b/src/json.hpp
index 0e14ec8d5e..7a8d76ae0f 100644
--- a/src/json.hpp
+++ b/src/json.hpp
@@ -146,6 +146,9 @@ struct has_mapped_type
         std::is_integral<decltype(detect(std::declval<T>()))>::value;
 };
 
+void to_json();
+void from_json();
+
 struct to_json_fn
 {
   private: