diff --git a/cpp/utils.cc b/cpp/utils.cc
index 592a48c..1ac7fa4 100644
--- a/cpp/utils.cc
+++ b/cpp/utils.cc
@@ -29,6 +29,9 @@ namespace f8
                 if (field_name.empty() || field_name[0] == '_') {
                     continue;
                 }
+                if (!reader.KeyExists(field_name) && field_desc->is_optional()) {
+                    continue;
+                }
 
                 switch (field_desc->cpp_type()) {
                 case google::protobuf::FieldDescriptor::CPPTYPE_STRING:
@@ -91,6 +94,9 @@ namespace f8
             if (field_name.empty() || field_name[0] == '_') {
                 continue;
             }
+            if (!jsonobj.HasKey(field_name) && field_desc->is_optional()) {
+                continue;
+            }
 
             if (field_desc->is_repeated()) {
                 std::shared_ptr<a8::XObject> repeated_field = jsonobj.At(field_name);