how to check invalid sample from sensor history

I have  a strange error on f6pro (but not in mine)  Error Name: System Error in line

x = MAT.round(v);

where v is data field from Toybox.SensorHistory.SensorSample

I suppose data/v is invalid but how to check it? Doc says:

The Sensor sample may contain invalid data. Make sure to has check for invalid samples. If the samples do not contain data, they will return null.

What has should I use to check sample? Now I check only null.