Statistics Sensor not working for a wunderground sensor


I have created two statistic sensors for two weather underground sensors:
temp_c and solarradiation. However only the temp_c statistic sensor is working.
solarradiation statistic sensor only has count and sample values, all the other are unknown.
here is the config:

  • platform: statistics
    entity_id: sensor.pws_temp_c
    name: “Stat.temp”
  • platform: statistics
    entity_id: sensor.pws_solarradiation
    name: “”

And here are the errors in the log
2018-08-21 18:52:37 ERROR (MainThread) [homeassistant.components.sensor.statistics] mean requires at least one data point
2018-08-21 18:52:38 ERROR (MainThread) [homeassistant.components.sensor.statistics] variance requires at least two data points

But the (weather underground) sensor has more data points, I can see it the history chart.

Am I missing something?