Group Sensor unknown state

Hi all,
This is my first post. i have been using HA from few months now and very grateful to developers for this wonderful software.
I have two Xiaomi Window Sensors which reports ‘on’ or ‘off’ state. I made a group ‘windowsensor’ and using automation to turn light on if any of these sensor state changed to ‘on’ from ‘off’. Everything was working fine but from some days I have noticed the group.windowsensor state is showing as ‘unknown’. I restarted HA and even upgraded to latest version but still issue is not resolved. Please help
HA Version 0.66.1 installed on raspbian strech.

Edit:- while posting this issue I figured that one of the sensor has mistake in spelling and fixed that resolved the issue.