passing a variable to cmd

[SS]

Note the following code:

import subprocess
import shlex

domname = raw_input("Enter your domain name: ");
print "Your domain name is: ", domname

print "\n"

# cmd='dig @4.2.2.2 nbc.com ns +short'
cmd="dig @4.2.2.2 %s ns +short", % (domname)
proc=subprocess.Popen(shlex.split(cmd),stdout=subprocess.PIPE)
out,err=proc.communicate()
print(out)

The line that is commented out works fine.  However, I want to substitute a variable for "nbc.com".  The command:

cmd="dig @4.2.2.2 %s ns +short", % (domname)

does not work.  I've tried different variations to no avail.  Any advice on how to get that variable working?

TIA.