apt-add-repository crashes trying to import key

I am running Ubuntu 12.04

Executing the next command: sudo add-apt-repository ppa:diesch/testing

leads to the next fatal error msg:
Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 551, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/dist-packages/softwareproperties/ppa.py", line 99, in run
    self.add_ppa_signing_key(self.ppa_path)
  File "/usr/lib/python2.7/dist-packages/softwareproperties/ppa.py", line 132, in add_ppa_signing_key
    tmp_keyring_dir = tempfile.mkdtemp()
  File "/usr/lib/python2.7/tempfile.py", line 322, in mkdtemp
    name = names.next()
  File "/usr/lib/python2.7/tempfile.py", line 141, in next
    letters = [choose(c) for dummy in "123456"]
  File "/usr/lib/python2.7/random.py", line 274, in choice
    return seq[int(self.random() * len(seq))] # raises IndexError if seq is empty
ValueError: cannot convert float NaN to integer

I have no idea what to do to solve this. When I afterwards run the command: apt-get update
I get the (relevant) msg:
W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5AF549300FEB6DD9