Help:“NaN ” occurs

[benm]

Cacti measures interface traffic (bits/sec) of 10 routers, but “NaN ” frequently occurs.
The packet was captured from the device, but there were no abnormal values in the data.
I can't fix it because the cause is not solved, so please tell me how to fix it if someone has experienced the same thing.

OS :RHEL8.2
Cacti :1.2.16
Spine :1.2.16
RRDtool :1.7
NET-SNMP:5.8
PHP :7.2.24
Mariadb :10.3.17

Thank you.