Failed to register for selective services. citizenship obstacle?

Many years ago, when my husband arrived to this country, he failed to register for selective services. Now that it is time to apply for his citizenship, turns out he has a big problem. We need to request the status information letter and give a plausible explanation of why he failed to register. The real explanation is simple - he had no idea about it. Did anyone have a similar situation? What would be an acceptable excuse in this situation?