iperf shows "Sent 0 datagrams"

I am running iperf from CLI on an ERX (iperf3 -c ping.online.net -u -b 4M) and after the ten test (which all look OK) I get the final summary and it shows "Sent 0 datagrams" with a transfer amount and bandwidth speed but under Lost/Total Datagrams it shows "0/0 (nan%)"

This works fine on other ERX's we have in the field.

This is an ERX v1.8.5

Any help would be apprecited.