When testing a zone transfer in Webmin’s BIND Test Zone Transfer function, you might end up with the following output:
Testing transfer of slave zone from .. .. from : Failed : /usr/bin/dig: couldn't get address for '': not found .. from : Failed : /usr/bin/dig: couldn't get address for '': not found
Solution
Go to Webmin’s website and download latest version:
https://www.webmin.com/download.html
If you had a similar issue than me, this will be the result of your upgrade:
Unpacking webmin (1.970) over (1.930)
References
This a a bug as per the link below: