Simple content with fixed value and attributes - xsd

Question

I have something along the lines of schema.xsd:
<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema" targetNamespace="http://www.example.com/" xml:lang="en">
<xsd:element name="Element" fixed="ElementValue">
<xsd:complexType>
<xsd:simpleContent>
<xsd:extension base="xsd:string">
<xsd:attribute name="attribute" type="xsd:string" use="required" fixed="AttributeValue"/>
</xsd:extension>
</xsd:simpleContent>
</xsd:complexType>
</xsd:element>
</xsd:schema>
and the corresponding document.xml:
<?xml version="1.0" ?>
<example:Element xmlns:example="http://www.example.com/" attribute="AttributeValue">ElementValue</example:Element>
Is this valid? I could not find anything in specs that would forbid such declaration, and xmllint validates it fine:
$ xmllint --schema schema.xsd document.xml --noout
document.xml validates
$ xmllint --schema http://www.w3.org/2001/XMLSchema.xsd schema.xsd --noout
schema.xsd validates
I did however see this question which seems to suggest that it is not correct, and the library I use (PyXB) doesn't seem to handle that:
>>> import example
>>> xml = open('document.xml').read()
>>> data = example.CreateFromDocument(xml)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "example.py", line 65, in CreateFromDocument
saxer.parse(io.BytesIO(xmld))
File "/usr/lib/python2.7/xml/sax/expatreader.py", line 110, in parse
xmlreader.IncrementalParser.parse(self, source)
File "/usr/lib/python2.7/xml/sax/xmlreader.py", line 123, in parse
self.feed(buffer)
File "/usr/lib/python2.7/xml/sax/expatreader.py", line 213, in feed
self._parser.Parse(data, isFinal)
File "/usr/lib/python2.7/xml/sax/expatreader.py", line 365, in end_element_ns
self._cont_handler.endElementNS(pair, None)
File "/usr/local/lib/python2.7/dist-packages/pyxb/binding/saxer.py", line 388, in endElementNS
binding_object = this_state.endBindingElement()
File "/usr/local/lib/python2.7/dist-packages/pyxb/binding/saxer.py", line 226, in endBindingElement
self.__constructElement(self.__delayedConstructor, self.__attributes, args)
File "/usr/local/lib/python2.7/dist-packages/pyxb/binding/saxer.py", line 116, in __constructElement
self.__bindingInstance = new_object_factory(*content, **kw)
File "/usr/local/lib/python2.7/dist-packages/pyxb/binding/basis.py", line 1631, in __call__
args = [ self.compatibleValue(args[0], **kw) ]
File "/usr/local/lib/python2.7/dist-packages/pyxb/binding/basis.py", line 1654, in compatibleValue
raise pyxb.ElementChangeError(self, value)
pyxb.exceptions_.ElementChangeError: Value ElementValue for element {http://www.example.com/}Element incompatible with fixed content
I would appreciate someone pointing me to the specs that explain why it's not correct, as I have to deal with a third-party XSD that uses that construct.