Python signxml "ValueError: Could not unserialize key data."

Question

I'm trying Python's signxml library to signing xml and verifying the signature. But it throws following exception, what could be wrong?

Traceback (most recent call last):
  File "C:/Users/digz/Documents/test_signxml/xml_verify_test_4.py", line 24, in <module>
    signed_root = XMLSigner().sign(root, key=key, cert=cert)
  File "C:\Python27\lib\site-packages\signxml\__init__.py", line 364, in sign
    key = load_pem_private_key(key, password=passphrase, backend=default_backend())
  File "C:\Python27\lib\site-packages\cryptography\hazmat\primitives\serialization.py", line 20, in load_pem_private_key
    return backend.load_pem_private_key(data, password)
  File "C:\Python27\lib\site-packages\cryptography\hazmat\backends\multibackend.py", line 289, in load_pem_private_key
    return b.load_pem_private_key(data, password)
  File "C:\Python27\lib\site-packages\cryptography\hazmat\backends\openssl\backend.py", line 1033, in load_pem_private_key
    password,
  File "C:\Python27\lib\site-packages\cryptography\hazmat\backends\openssl\backend.py", line 1202, in _load_key
    self._handle_key_loading_error()
  File "C:\Python27\lib\site-packages\cryptography\hazmat\backends\openssl\backend.py", line 1274, in _handle_key_loading_error
    raise ValueError("Could not unserialize key data.")
ValueError: Could not unserialize key data.

Here's the code I'm using:

from xml.etree import ElementTree
from signxml import XMLSigner, XMLVerifier
from myapp import settings

cert = open(settings.STATIC_ROOT + '/file/public.cert', 'rt').read()
key = open(settings.STATIC_ROOT + '/file/public.key', 'rt').read()

root = ElementTree.fromstring('<xml1>12</xml1>')
signed_root = XMLSigner().sign(root, key=key, cert=cert)
verified_data = XMLVerifier().verify(signed_root).signed_xml
print verified_data