significance of -1 when polling for temp

I've set up a polling process for my CT-80 that polls the current temperature and humidity every minute. This generally works, although occasionally, the temperature reports back at -1 degrees. (Humidity is always in the expected range.) Is there any special significance to this return value? To give an idea of the frequency of occurrence, it happened five times out of 2,000 samples taken over the last day and a half.