An unusual chain of events led to reports of a "cash register on fire" at a Santa Rosa market Tuesday.

When firefighters arrived at the Community Market on Mendocino Avenue south of Steele Lane at 5 p.m., they found the market filling with black, sooty smoke.

A plastic computer keyboard was smoldering near the cash register in what first appeared to be an electrical malfunction, Santa Rosa Fire Department Battalion Chief Jack Piccinini said.

But upon closer examination, firefighters discovered that the evening sun had struck a magnifying glass hanging near the register at just the right angle to cause a plastic coin cup to melt and catch fire.

That caught the keyboard on fire, Piccinini said.

Security video confirmed the magnifying glass was the culprit.

The store was closed, but a group of teens hanging out nearby on a picnic bench reported the smoke, as did a woman walking by.

Firefighters were able to flush out the smoke and the store will be open today. Other than the keyboard, there was no damage.