The machine I'm running netscanner on has its IP address/mask set to 192.168.5.1/22. The CIDR subnet for this is 192.168.4.0/22 but netscanner defaulted the subnet to 192.168.5.0/24 when this interface was selected.
Once I manually changed the subnet to 192.168.4.0/22 everything worked as expected.
Netscanner should be able to deduce the subnet address from the network configuration information.