strange!!! Xiaobaitu and other people, please help!!!

I got my GC in May. But yesterday I got a mail from local immigration office. It's a Fingerprint Notification. It said that I need do fingerprint for my I485 approvement.
What am I going to do? I want to ignore the letter, Because it's obviously their mistake. But I am a little worried.
Xiaobaitu, please help!!! Other people who had the same experience, please help me!!!
Thank you very much.