SAN FRANCISCO (CBS SF) — A portable toilet was set on fire Thursday evening near Lake Merced, according to San Francisco police.

The fire was reported around 6 p.m. Thursday at Lake Merced and Sunset boulevards, according to police.

READ MORE: Emeryville Tax Preparer Accused Of Filing Dozens Of Fraudulent Returns, Teaching Fraud In 'Tax School'

The San Francisco Fire Department put out the blaze, and fire officials believe it was the work of an arsonist, police said. No injuries were reported.

Two portable toilets were also set on fire in the Richmond District at Geary Boulevard and 32nd Avenue in the early morning hours on Sept. 7.

READ MORE: Liana Wallace On 'Survivor' All-Black Alliance Falling Apart: 'Just Wanted Us To Make Top 8, Then We Can Have World War II''

Police said there is no indication the two incidents are related.

Portable toilet fires are not an uncommon occurrence in the city. In the fall and winter of 2008-09, 27 portable toilets were burned throughout the city; the next year, several more went up in flames.

No injuries were reported in any of those fires, but the fire department warned at the time that the blazes had the potential to spread to nearby trees and homes. The lavatories also cost about $2,500 apiece.

MORE NEWS: Man Wins $1 Million On Scratch-Off Lottery Ticket Gifted To Him After Heart Surgery

(Copyright 2012 by CBS San Francisco and Bay City News Service. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.)