diff --git a/example_app/openai_app/lib/bloc/openai/openai_bloc.dart b/example_app/openai_app/lib/bloc/openai/openai_bloc.dart
index e71d47b..68c9203 100644
--- a/example_app/openai_app/lib/bloc/openai/openai_bloc.dart
+++ b/example_app/openai_app/lib/bloc/openai/openai_bloc.dart
@@ -127,17 +127,17 @@ class OpenAIBloc extends Cubit<OpenAIState> {
         .transform(StreamTransformer.fromHandlers(handleError: handleError))
         .listen((it) {
       Message? message;
-      for (final m in list) {
-        if (m.id == '${it.id}') {
-          message = m;
-          list.remove(m);
-          break;
+      list.removeWhere((element){
+        if(element.id == '${it.id}'){
+          message = element;
+          return true;
         }
-      }
+        return false;
+      });
 
       ///+= message
       message?.message =
-          '${message.message ?? ""}${it.choices.last.message?.content ?? ""}';
+          '${message?.message ?? ""}${it.choices.last.message?.content ?? ""}';
       list.add(Message(isBot: true, id: '${it.id}', message: message?.message));
       emit(ChatCompletionState(
           isBot: true, messages: list, showStopButton: true));
diff --git a/lib/src/model/complete_text/response/usage.dart b/lib/src/model/complete_text/response/usage.dart
index 4d8f75d..2829a64 100644
--- a/lib/src/model/complete_text/response/usage.dart
+++ b/lib/src/model/complete_text/response/usage.dart
@@ -1,17 +1,15 @@
 class Usage {
-  final int promptTokens;
-  final int completionTokens;
-  final int totalTokens;
+  final int? promptTokens;
+  final int? completionTokens;
+  final int? totalTokens;
   final String id = "${DateTime.now().millisecondsSinceEpoch}";
 
   Usage(this.promptTokens, this.completionTokens, this.totalTokens);
 
   factory Usage.fromJson(Map<String, dynamic> json) => Usage(
-        json['prompt_tokens'] as int,
-        json['completion_tokens'] == null
-            ? 0
-            : json['completion_tokens'] as int,
-        json['total_tokens'] as int,
+        json['prompt_tokens'],
+        json['completion_tokens'],
+        json['total_tokens'],
       );
   Map<String, dynamic> toJson() => usageToJson(this);