Issue with Lock custom signup

I’ve got an issue with lock custom login.
I added some additional fields to my custom login like this:

Then I tried to configure, from saml add-on page, the way my data are mapped, in order to pass the extra data to my application.
After that I click on “debug”, do a test sign up and get the resulting xml.
The issue here is that the xml contains an “[object object]” instead of the data I have configured:
<saml:Attribute Name="[object Object]" NameFormat=“urn:oasis:names:tc:SAML:2.0:attrname-format:unspecified”>
<saml:AttributeValue xsi:type=“xs:anyType”>[object Object]</saml:AttributeValue>
I checked and when I save the configuration in the saml add-on page, the data are sent correctly, serialized.
Do you know what I’m missing?