覆盖的错误消息未在序列化器中显示

Question

我有一个ListField序列化器，如果为空，它应该显示相应的消息：

class CreatePostSerializer(serializers.ModelSerializer):
    user = serializers.PrimaryKeyRelatedField(
        read_only=True,
    )

    topics = serializers.ListField(
        child=serializers.CharField(
            max_length=50,
            error_messages={'max_length': 'Each tag should be no more than 50 characters.'}
        ),
        max_length=3,
        write_only=True,
        error_messages={'required': 'You need to add at least one tag to your question.'}
    )

    class Meta:
        model = Question
        fields = ('user', 'status', 'topics')

当列表为空时，未引发错误消息：You need to add at least one tag to your question.，而是引发了默认错误消息：`此字段为必填。”如何使其显示我的自定义错误消息代替？