Run this in one shell on the system:

[root@localhost ~]# ping6 fe80::1%eth0
PING fe80::1%eth0(fe80::1) 56 data bytes
From fe80::5054:ff:fe75:31cc icmp_seq=1 Destination unreachable: Address unreachable
…

Now start ndptool in another shell:

[root@localhost ~]# ndptool monitor
ndp eventfd handler call failed
Command failed "No such file or directory"
[root@localhost ~]# 

Expected behavior is that ndptool continues to run, reporting neighbor discovery data as it is encountered.