"XML or text declaration not at start of entity: line 2, column 0" when calling ElementTree.parse

ElementTree.parse() fails in the simple example below with the error

xml.etree.ElementTree.ParseError: XML or text declaration not at start of entity: line 2, column 0

The XML looks valid and the code is simple, so what am I doing wrong?

xmlExample = """
<?xml version="1.0"?>
<data>
    stuff
</data>
"""
import io
source = io.StringIO(xmlExample)
import xml.etree.ElementTree as ET
tree = ET.parse(source)