diff --git a/tdtl/td/tl/tl_simple.h b/tdtl/td/tl/tl_simple.h
index dd5a1e53..06d6ea13 100644
--- a/tdtl/td/tl/tl_simple.h
+++ b/tdtl/td/tl/tl_simple.h
@@ -81,12 +81,16 @@ struct Type {
 struct Arg {
   const Type *type;
   std::string name;
+  int var_num = -1;
+  int exist_var_num = -1;
+  int exist_var_bit = -1;
 };
 
 struct Constructor {
   std::string name;
   std::int32_t id;
   std::vector<Arg> args;
+  int var_count = 0;
   const CustomType *type;
 };
 
@@ -100,6 +104,7 @@ struct CustomType {
 
 struct Function {
   std::string name;
+  int var_count = 0;
   std::int32_t id;
   std::vector<Arg> args;
   const Type *type;
@@ -248,11 +253,15 @@ class Schema {
       constructor = constructors_.back().get();
       constructor->id = from->id;
       constructor->name = from->name;
+      constructor->var_count = from->var_count;
       constructor->type = get_custom_type(config_->get_type(from->type_id));
       for (auto &from_arg : from->args) {
         Arg arg;
         arg.name = from_arg.name;
         arg.type = get_type(from_arg.type);
+        arg.var_num = from_arg.var_num;
+        arg.exist_var_num = from_arg.exist_var_num;
+        arg.exist_var_bit = from_arg.exist_var_bit;
         constructor->args.push_back(std::move(arg));
       }
     }
@@ -266,11 +275,15 @@ class Schema {
       function = functions_.back().get();
       function->id = from->id;
       function->name = from->name;
+      function->var_count = from->var_count;
       function->type = get_type(config_->get_type(from->type_id));
       for (auto &from_arg : from->args) {
         Arg arg;
         arg.name = from_arg.name;
         arg.type = get_type(from_arg.type);
+        arg.var_num = from_arg.var_num;
+        arg.exist_var_num = from_arg.exist_var_num;
+        arg.exist_var_bit = from_arg.exist_var_bit;
         function->args.push_back(std::move(arg));
       }
     }
diff --git a/tl/generate/tl_json_converter.cpp b/tl/generate/tl_json_converter.cpp
index a0213ef5..d1a0488f 100644
--- a/tl/generate/tl_json_converter.cpp
+++ b/tl/generate/tl_json_converter.cpp
@@ -48,13 +48,30 @@ void gen_to_json_constructor(StringBuilder &sb, const T *constructor, bool is_he
   sb << " {\n";
   sb << "  auto jo = jv.enter_object();\n";
   sb << "  jo(\"@type\", \"" << constructor->name << "\");\n";
+  std::vector<std::string> var_names(constructor->var_count);
+  for (auto &arg : constructor->args) {
+    if (arg.var_num >= 0) {
+      CHECK(arg.var_num < (int)var_names.size());
+      var_names[arg.var_num] = tl::simple::gen_cpp_field_name(arg.name);
+    }
+  }
   for (auto &arg : constructor->args) {
     auto field_name = tl::simple::gen_cpp_field_name(arg.name);
-    // TODO: or as null
-    bool is_custom = arg.type->type == tl::simple::Type::Custom;
+    bool is_optional = arg.type->type == tl::simple::Type::Custom || arg.exist_var_num >= 0;
 
-    if (is_custom) {
-      sb << "  if (object." << field_name << ") {\n  ";
+    if (is_optional) {
+      sb << "  if (";
+      if (arg.type->type == tl::simple::Type::Custom) {
+        sb << "object." << field_name;
+        if (arg.exist_var_num >= 0) {
+          sb << " && ";
+        }
+      }
+      if (arg.exist_var_num >= 0) {
+        CHECK(arg.exist_var_num < (int)var_names.size());
+        sb << "(object." << var_names[arg.exist_var_num] << " & " << (1 << arg.exist_var_bit) << ")";
+      }
+      sb << ") {\n  ";
     }
     auto object = PSTRING() << "object." << tl::simple::gen_cpp_field_name(arg.name);
     if (arg.type->type == tl::simple::Type::Bytes || arg.type->type == tl::simple::Type::SecureBytes) {
@@ -72,7 +89,7 @@ void gen_to_json_constructor(StringBuilder &sb, const T *constructor, bool is_he
       object = PSTRING() << "JsonVectorInt64{" << object << "}";
     }
     sb << "  jo(\"" << arg.name << "\", ToJson(" << object << "));\n";
-    if (is_custom) {
+    if (is_optional) {
       sb << "  }\n";
     }
   }