DIAGNOSIS The float switch inside the reservoir compartment is stuck or the sensor is faulty. This is the white plastic bobber on the left side.
FIX Remove the water reservoir. Manually move the float up and down several times with your finger to ensure it's not stuck on its post. Also, check that the little magnet on the bottom of the float is intact and clean.
IF THAT DIDN'T WORK Look inside the empty reservoir slot. There's a small magnetic reed switch on the left wall. Wipe it with a dry cotton swab to remove any dust or mineral deposits that could be interfering with the signal.
HEADS UP Don't overfill the reservoir past the "Max" line. Water can splash into the float compartment and cause it to stick or give false readings.