diff --git a/lib/src/serialize.dart b/lib/src/serialize.dart
index 5c7beee..d4c9fab 100644
--- a/lib/src/serialize.dart
+++ b/lib/src/serialize.dart
@@ -614,6 +614,8 @@ void serializeStruct(Type self, SerialBuffer buffer, Object data,
     } else {
       if (allowExtensions && field.type.extensionOf != null) {
         state.skippedBinaryExtension = true;
+      } else if(field.type.optionalOf != null ) {
+        field.type.serialize(field.type, buffer, dy[field.name],state: state);
       } else {
         throw 'missing ' +
             self.name +