[Python] Failed to sign xmlsec with named reference URI

I tried to use python-xmlsec to sign an xml document from a template. It works fine if URI="". However, if URI is not empty, then I got error "failed to sign"

python 3.6 xmlsec from https://github.com/mehcode/python-xmlsec

import xmlsec

xml_in='''<Envelope xmlns="urn:envelope">
<Data ID="1234">
Hello, World!
</Data>
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
<SignedInfo>
<CanonicalizationMethod Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315" />
<SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1" />
<Reference URI="#1234">
<Transforms>
<Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" />
</Transforms>
<DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" />
<DigestValue></DigestValue>
</Reference>
</SignedInfo>
<SignatureValue/>
<KeyInfo>
<KeyName/>
</KeyInfo>
</Signature>
</Envelope>'''

template=etree.ElementTree(etree.fromstring(xml_in)).getroot()

signature_node = xmlsec.tree.find_node(template, xmlsec.constants.NodeSignature)

ctx = xmlsec.SignatureContext()

ctx.key = xmlsec.Key.from_file('c:/certificates/ercot.key', xmlsec.constants.KeyDataFormatPem)

ctx.sign(signature_node)

print(etree.tostring(template).decode())


ERROR below:

Traceback (most recent call last):

File "<ipython-input-243-4de30b62be4f>", line 5, in <module>
ctx.sign(signature_node)

Error: (1, 'failed to sign')```

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