diff --git a/lib/http/converter/id_converter.dart b/lib/http/converter/id_converter.dart
index e51aad2..fb01f48 100644
--- a/lib/http/converter/id_converter.dart
+++ b/lib/http/converter/id_converter.dart
@@ -1,5 +1,6 @@
 
 import 'package:jmap_dart_client/jmap/core/id.dart';
+import 'package:jmap_dart_client/jmap/core/reference_id.dart';
 import 'package:json_annotation/json_annotation.dart';
 
 class IdConverter implements JsonConverter<Id, String> {
@@ -12,7 +13,11 @@ class IdConverter implements JsonConverter<Id, String> {
 
   @override
   String toJson(Id object) {
-    return object.value;
+    if (object is ReferenceId) {
+      return object.toString();
+    } else {
+      return object.value;
+    }
   }
 
 }
\ No newline at end of file