dnspython update dns record issue

Thu Mar 2 13:52:35 EST 2017

I'm trying to use dnspython to do a TSIG based dynamic DNS update.
I've followed the example code :
#!/usr/bin/env python
import sys
import dns.update
import dns.query
import dns.tsigkeyring

keyring = dns.tsigkeyring.from_text({
 'keyname.' : 'VarFF=2Xts7T5Vb/xd&8ir=='
})
update = dns.update.Update('domain_name', keyring=keyring)
update.replace('myserver', 30, 'a', '88.88.88.88')
response = dns.query.tcp(update, 'dns_ip',timeout=10)
and here is what I got:
 response = dns.query.tcp(update, 'dns_ip',timeout=10)
 File "/root/bck/dnspython/dns/query.py", line 475, in tcp
 q.keyring, q.mac)
 File "/root/bck/dnspython/dns/query.py", line 412, in receive_tcp
 one_rr_per_rrset=one_rr_per_rrset)
 File "/root/bck/dnspython/dns/message.py", line 821, in from_wire
 reader.read()
 File "/root/bck/dnspython/dns/message.py", line 749, in read
 self._get_section(self.message.additional, adcount)
 File "/root/bck/dnspython/dns/message.py", line 701, in _get_section
 self.message.first)
 File "/root/bck/dnspython/dns/tsig.py", line 198, in validate
 raise BadSignature
dns.tsig.BadSignature: The TSIG signature fails to verify.
Thanks in advance to anyone who can help!