diff --git a/src/json.hpp b/src/json.hpp
index 109691380..1e3c6f3b4 100644
--- a/src/json.hpp
+++ b/src/json.hpp
@@ -289,6 +289,8 @@ struct is_compatible_basic_json_type
                                  T>::value;
 };
 
+
+// This trait checks if JSONSerializer<T>::from_json exists
 template <template <typename, typename> class JSONSerializer, typename Json,
           typename T>
 struct has_from_json
@@ -305,6 +307,7 @@ public:
       detect(std::declval<JSONSerializer<T, void>>()))>::value;
 };
 
+// This trait checks if JSONSerializer<T>::to_json exists
 template <template <typename, typename> class JSONSerializer, typename Json,
           typename T>
 struct has_to_json
@@ -322,7 +325,7 @@ public:
 };
 
 // those declarations are needed to workaround a MSVC bug related to ADL
-// (idea taken from MSVC-Ranges implementation
+// (taken from MSVC-Ranges implementation)
 void to_json();
 void from_json();
 
diff --git a/test/src/unit-udt.cpp b/test/src/unit-udt.cpp
index 826baaae3..800f9868e 100644
--- a/test/src/unit-udt.cpp
+++ b/test/src/unit-udt.cpp
@@ -397,6 +397,29 @@ struct my_serializer
   }
 };
 
+// partial specialization on optional_type
+template <typename T>
+struct my_serializer<udt::optional_type<T>>
+{
+  template <typename Json>
+  static void from_json(Json const& j, udt::optional_type<T>& opt)
+  {
+    if (j.is_null())
+      opt = nullptr;
+    else
+      opt = j.get<T>();
+  }
+
+  template <typename Json>
+  static void to_json(Json& j, udt::optional_type<T> const& opt)
+  {
+    if (opt)
+      j = *opt;
+    else
+      j = nullptr;
+  }
+};
+
 using my_json = nlohmann::basic_json<std::map, std::vector, std::string, bool,
                                      std::int64_t, std::uint64_t, double,
                                      std::allocator, my_serializer>;
@@ -414,7 +437,7 @@ namespace udt
   }
 }
 
-TEST_CASE("custom serializer")
+TEST_CASE("custom serializer", "[udt]")
 {
   SECTION("default use works like default serializer")
   {
@@ -429,4 +452,17 @@ TEST_CASE("custom serializer")
     CHECK(pod2 == pod3);
     CHECK(pod2 == pod);
   }
+
+  SECTION("serializer specialization")
+  {
+    udt::optional_type<int> opt;
+
+    json j{opt};
+    CHECK(j.is_null());
+
+    opt = 42;
+    j = json{opt};
+    CHECK(j.get<udt::optional_type<int>>() == opt);
+    CHECK(42 == j.get<int>());
+  }
 }