nbd-client error message unclear when failing to disconnect from server

Ubuntu version:
Description: Ubuntu 17.04
Release: 17.04

Package version:
nbd-client:
  Installed: 1:3.15.1-2
  Candidate: 1:3.15.1-2
  Version table:
 *** 1:3.15.1-2 500
        500 http://gb.archive.ubuntu.com/ubuntu zesty/universe amd64 Packages
        100 /var/lib/dpkg/status

Steps to reproduce:
-Use nbd-client 3.15.1-2 (Zesty)
-Connect to nbd server and read from host (i.e. sudo nbd-client dt56.xenrt.citrite.net /dev/nbd2 -N "")
-Attempt to connect again to server without explicitly disconnecting
-Connection fails and returns the following error message:
------
root@thomasmc-VirtualBox:~/Documents# sudo nbd-client dt56.xenrt.citrite.net /dev/nbd2 -N ""
Warning: the oldstyle protocol is no longer supported.
This method now uses the newstyle protocol with a default export
Negotiation: ..size = 0MB
bs=1024, sz=0 bytes
Error: Ioctl NBD_SET_SOCK failed: Invalid argument

Exiting.
------

Error message does not make it clear that error occured because I failed to disconnect. This is particularly confusing as with nbd-client 3.12 I was able to connect repeatedly without the need for an explicit disconnect.

What I expected to happen:
Either for the second connection to pass (as it does with nbd-cliebt 3.12) or if its going to fail for it to make clear that the failure is because I failed to disconnect.

What happened instead:
Connection failed and passed an unhelpful error message.