Python + DNS : Cannot get RRSIG records: No Answer

>>> import dns.resolver
>>> myresolver = dns.resolver.Resolver()
>>> myresolver.use_edns(1, 0, 1400)
>>> print myresolver.query('sources.org', 'DNSKEY')
<dns.resolver.Answer object at 0xb78ed78c>
>>> print myresolver.query('ripe.net', 'NSEC')
<dns.resolver.Answer object at 0x8271c0c>

>>> print myresolver.query('sources.org', 'RRSIG')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.5/site-packages/dns/resolver.py", line 664, in query
    answer = Answer(qname, rdtype, rdclass, response)
  File "/usr/lib/python2.5/site-packages/dns/resolver.py", line 121, in __init__
    raise NoAnswer

16:09:39.342532 IP 192.134.4.69.53381 > 192.134.4.162.53: 22330+ [1au] RRSIG? sources.org. (40)
16:09:39.343229 IP 192.134.4.162.53 > 192.134.4.69.53381: 22330 8/5/6 RRSIG[|domain]

>>> print myresolver.query('sources.org', 'RRSIG', 'ANY')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.5/site-packages/dns/resolver.py", line 664, in query
    answer = Answer(qname, rdtype, rdclass, response)
  File "/usr/lib/python2.5/site-packages/dns/resolver.py", line 121, in __init__
    raise NoAnswer
dns.resolver.NoAnswer