why hcsr04 senser distance sometimes -1?

[tobecrazy]

I use the same code as simple code ,I didn't change the distance of the sensor.
get below logs :
019-12-19 20:17:24.972 300-462/system_process D/WificondControl: Scan result ready event
2019-12-19 20:17:26.985 2233-2233/cn.dbyl.study I/CarSys: sensor changed: [-1.000000]
2019-12-19 20:17:28.996 2233-2233/cn.dbyl.study I/CarSys: sensor changed: [12.862785]
2019-12-19 20:17:31.009 2233-2233/cn.dbyl.study I/CarSys: sensor changed: [-1.000000]
2019-12-19 20:17:33.023 2233-2233/cn.dbyl.study I/CarSys: sensor changed: [11.111112]
2019-12-19 20:17:35.033 2233-2233/cn.dbyl.study I/CarSys: sensor changed: [75.510902]
2019-12-19 20:17:35.035 2233-2233/cn.dbyl.study I/CarSys: sensor changed: [74.051178]