With no visible signs, sounds like you might be able to contest it.

I live in a relatively new neighborhood with no posted signs regarding the city's ordinance on overnight parking. We have one Savage city street and the rest are association/neighborhood streets that are governed by our association's rules. Guest parking is insufficient and many park on the street overnight during non-winter seasons. A couple months back, the police were called by a neighbor who was annoyed by the parking. A car was sent out to make a sweep and > 20 people were ticketed. I wasn't included as I have no problem putting my car in a garage and can usually squeeze friends cars into the driveway.

The city does not have to post signs; residents and visitors are expected to know and abide by the city's ordinances. Unfortunately for new residents, that means surfing the antiquated city web page or going to city hall and requesting them.

I'll give you that ignorance is not a defense. 99% of the time it will get you nowhere. However, in this case it worked. When you have 20+ people taken by surprise and angry/disappointed it can get some attention. Neighbors were encouraged to challenge the tickets and all were reversed.

We were told they will not come out looking for parking violations, but will respond to complaints and may ticket cars they see in violation while out on another call. They are looking at installing a sign as well.

My advice - challenge the fine, apologize and find somewhere else to park in the future. Request that the street be signed, particularly if it was in a neighborhood. It can be difficult to tell if you are on a city street or one owned/maintained by a neighborhood. That leads to more confusion and a better chance of getting out of a ticket.