It happens only with DebugLevel=4.

Zabbix agent tries to send a message to Zabbix using:
process_value(server, port, CONFIG_HOSTNAME, active_metrics[i].key_orig, NULL,...)
5-th parameter is value.

and in process_value:
zabbix_log(LOG_LEVEL_DEBUG, "In %s() key:'%s:%s' value:'%s'", __function_name, host, key, value);

but value variable can not be NULL in this case.