ElementTree does not write data in UTF-8

I'm pulling in data from a database and attempting to create an XML file from this data. The data is in UTF-8 and can contain characters such as á, š, or č. This is the code:

import xml.etree.cElementTree as ET

tree = ET.parse(metadata_file)
# ..some commands that alter the XML..
tree.write(metadata_file, encoding="UTF-8")

When writing the data, the script fails with:

Traceback (most recent call last):
  File "get-data.py", line 306, in <module>
    main()
  File "get-data.py", line 303, in main
    tree.write(metadata_file, encoding="UTF-8")
  File "/usr/lib64/python2.7/xml/etree/ElementTree.py", line 820, in write
    serialize(write, self._root, encoding, qnames, namespaces)
  File "/usr/lib64/python2.7/xml/etree/ElementTree.py", line 939, in _serialize_xml
    _serialize_xml(write, e, encoding, qnames, None)
  File "/usr/lib64/python2.7/xml/etree/ElementTree.py", line 939, in _serialize_xml
    _serialize_xml(write, e, encoding, qnames, None)
  File "/usr/lib64/python2.7/xml/etree/ElementTree.py", line 937, in _serialize_xml
    write(_escape_cdata(text, encoding))
  File "/usr/lib64/python2.7/xml/etree/ElementTree.py", line 1073, in _escape_cdata
    return text.encode(encoding, "xmlcharrefreplace")
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 32: ordinal not in range(128)

The only way to prevent this is to decode the data written to the XML file with:

text = text.decode('utf-8')

but then the resulting file will contain e.g. č rather than a č. Any idea how I can write the data to the file and keep it in UTF-8?