[bug] Server error on creating IPv4 subnet from IPv6 master subnet

[pandafy]

Steps to Replicate

1. Create a an IPv6 subnet as shown in the screenshot below
   
2. Create a new IPv4 subnet but use the previously created IPv6 subnet for master subnet as shown in the screenshot
   

Expected outcome
On saving the IPv4 subnet in step 2, the UI should show an error that the desired IPv4 subnet cannot be created for IPv6 master suibnet.

Actual outcome
Server error