Fire damaged a Napa guest house early Monday, but did not injure anyone, Cal Fire reports.

Firefighters were called to a home in the 1100 block of 1st Avenue around 2:21 a.m. and found the nearby guest house on fire. Units from Cal Fire and the city and county fire departments fought the blaze for about an hour.

The cause remains under investigation, Cal Fire said, and it's not yet clear how much damage the structure sustained . It's also not yet clear whether the guest house was occupied at the time.