iperf3: error - control socket has closed unexpectedly v.3.6

# Context

* Version of iperf3: 3.6

* Hardware: raspberry pi 3 b

* Operating system (and distribution, if any): Debian Linux (default pi OS)

# Bug Report
I'm trying to run iperf3.6 over a 100kbps link to measuer the actual throughput. I'm running it on two pis.
On the server, I run "iperf3 -s --logfile /dev/tty
On the client, I run "iperf3 -c (IPv6 address of peer) -u -b 390k -l 1024 -t 12"

* Expected Behavior I see 12 reports, one second apart, then the summary

* Actual Behavior, on the client I see 11 reports, one second apart, then a pause, then iperf3: error - control socket has closed unexpectedly. On the server I see 16 reports one second apart then iperf3: error - select failed: Bad file descriptor

* Steps to Reproduce: given above

Server side:
-----------------------------------------------------------
Server listening on 5201
-----------------------------------------------------------
Accepted connection from (IP addr), port 56688
[ 6] local (ip addr) port 5201 connected to (ip addr) port 54260
[ ID] Interval Transfer Bitrate Jitter Lost/Total Datagrams
[ 6] 0.00-1.00 sec 9.00 KBytes 73.7 Kbits/sec 37.226 ms 0/9 (0%)
[ 6] 1.00-2.00 sec 10.0 KBytes 81.9 Kbits/sec 55.150 ms 0/10 (0%)
[ 6] 2.00-3.00 sec 11.0 KBytes 90.1 Kbits/sec 65.500 ms 0/11 (0%)
[ 6] 3.00-4.00 sec 9.00 KBytes 73.7 Kbits/sec 72.033 ms 0/9 (0%)
[ 6] 4.00-5.00 sec 11.0 KBytes 90.1 Kbits/sec 73.836 ms 0/11 (0%)
[ 6] 5.00-6.00 sec 10.0 KBytes 81.9 Kbits/sec 74.872 ms 0/10 (0%)
[ 6] 6.00-7.00 sec 11.0 KBytes 90.1 Kbits/sec 74.872 ms 0/11 (0%)
[ 6] 7.00-8.00 sec 10.0 KBytes 81.9 Kbits/sec 75.292 ms 0/10 (0%)
[ 6] 8.00-9.00 sec 10.0 KBytes 81.9 Kbits/sec 75.182 ms 0/10 (0%)
[ 6] 9.00-10.00 sec 11.0 KBytes 90.1 Kbits/sec 75.403 ms 0/11 (0%)
[ 6] 10.00-11.00 sec 10.0 KBytes 81.9 Kbits/sec 89.071 ms 26/36 (72%)
[ 6] 11.00-12.00 sec 10.0 KBytes 81.9 Kbits/sec 53.561 ms 38/48 (79%)
[ 6] 12.00-13.00 sec 10.0 KBytes 81.9 Kbits/sec 36.800 ms 36/46 (78%)
[ 6] 13.00-14.00 sec 10.0 KBytes 81.9 Kbits/sec 25.029 ms 35/45 (78%)
[ 6] 14.00-15.00 sec 11.0 KBytes 90.1 Kbits/sec 17.210 ms 40/51 (78%)
[ 6] 15.00-16.00 sec 10.0 KBytes 81.9 Kbits/sec 13.892 ms 36/46 (78%)
iperf3: error - select failed: Bad file descriptor
-----------------------------------------------------------
Server listening on 5201
-----------------------------------------------------------

Client side:
Connecting to host (IP addr) port 5201
[ 5] local (IP addr) port 54260 connected to (ip addr) port 5201
[ ID] Interval Transfer Bitrate Total Datagrams
[ 5] 0.00-1.00 sec 48.0 KBytes 393 Kbits/sec 48
[ 5] 1.00-2.00 sec 48.0 KBytes 393 Kbits/sec 48
[ 5] 2.00-3.00 sec 47.0 KBytes 385 Kbits/sec 47
[ 5] 3.00-4.00 sec 48.0 KBytes 393 Kbits/sec 48
[ 5] 4.00-5.00 sec 47.0 KBytes 385 Kbits/sec 47
[ 5] 5.00-6.00 sec 48.0 KBytes 393 Kbits/sec 48
[ 5] 6.00-7.00 sec 48.0 KBytes 393 Kbits/sec 48
[ 5] 7.00-8.00 sec 47.0 KBytes 385 Kbits/sec 47
[ 5] 8.00-9.00 sec 48.0 KBytes 393 Kbits/sec 48
[ 5] 9.00-10.00 sec 48.0 KBytes 393 Kbits/sec 48
[ 5] 10.00-11.00 sec 47.0 KBytes 385 Kbits/sec 47
iperf3: error - control socket has closed unexpectedly