公民配偶婚姻绿卡转正 I-751 要不要打指纹的钱？

我是公民，老婆的两年绿卡转正，751 被退了回来。说支票不对。我看了751 instruction 说

U.S. Citizen or Lawful Permanent Resident Spouse. If you are the U.S. citizen or lawful permanent resident spouse, you do not need to include a biometric services fee at the time this petition is submitted. If USCIS later notifies you that you must submit your biometrics, you will receive a biometric services appointment notice with instructions on how to submit the additional fee.

我的理解是公民配偶缴费不用交指纹费用。这个理解对吗？ 

谢谢，