uburst log headed with 0s

we have uburst monitoring enabled, but our logs always seem to get headed with a pile of NULs. It seems to be being written as a sparse file so the file doesn't take up much disk, but it's awfully hard to parse. this is with 3.1.180226. 

--interface fbcard:0:a:00
-P=/var/run/n2disk-fbcard:0:a:00.pid
-s 9000
--chunk-len 128
--buffer-len 4096
--dump-directory /srv/pcaps/7ticks-A/dumps
--max-file-len 512
--max-file-duration 900
--file-prefix fbcard:0:a:0-
--max-nested-dirs 10
--max-num-files 1000
--hugepages
--timeline-dir /srv/pcaps/7ticks-A/timeline
--reader-cpu-affinity 10
--writer-cpu-affinity 12
--compressor-cpu-affinity 32,34,36,38
--uburst-detection
--uburst-log /var/log/n2disk/uburst-fbcard:0:a:0.log
--uburst-win-size 100
--uburst-link-speed 10000
--uburst-threshold 80
--pcap-compression
--cluster-id 1
--index
--index-on-compressor-threads
--nanoseconds

any thoughts?