Error when using Adobe Sign in Workday

Good morning,

I would like to have your insight in an error.

We are receiving the following error when trying to review and sign a document in Workday:

Error message:
Validation error occurred. Cannot resolve instance from Workday Id if id is null
Exception:
BadRequestException :
Error code : unknown
Response :<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/">
<SOAP-ENV:Body>
<SOAP-ENV:Fault xmlns:wd="urn:com.workday/bsvc">
<faultcode>SOAP-ENV:Client.validationError</faultcode>
<faultstring>Validation error occurred. Cannot resolve instance from Workday Id if id is null</faultstring>
<detail>
<wd:Validation_Fault>
<wd:Validation_Error>
<wd:Message>Cannot resolve instance from Workday Id if id is null</wd:Message>
<wd:Detail_Message>Cannot resolve instance from Workday Id if id is null. Cause: java.lang.NullPointerException: Cannot resolve instance from Workday Id if id is null</wd:Detail_Message>
<wd:Xpath>/bsvc:Put_eSignature_Integration_Data_Request[1]/bsvc:eSignature_Signing_URL_Data[1]/bsvc:eSignature_Document_Signing_URL_Data[1]/bsvc:Person_Reference[1]</wd:Xpath>
</wd:Validation_Error>
</wd:Validation_Fault>
</detail>
</SOAP-ENV:Fault>
</SOAP-ENV:Body>
</SOAP-ENV:Envelope>
com.capeclear.capeconnect.transport.BadRequestException: error from returned HTTP status: 500 <SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/">
<SOAP-ENV:Body>
<SOAP-ENV:Fault xmlns:wd="urn:com.workday/bsvc" xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/">
<faultcode>SOAP-ENV:Client.validationError</faultcode>
<faultstring>Validation error occurred. Cannot resolve instance from Workday Id if id is null</faultstring>
<detail>
<wd:Validation_Fault>
<wd:Validation_Error>
<wd:Message>Cannot resolve instance from Workday Id if id is null</wd:Message>
<wd:Detail_Message>Cannot resolve instance from Workday Id if id is null. Cause: java.lang.NullPointerException: Cannot resolve instance from Workday Id if id is null</wd:Detail_Message>
<wd:Xpath>/bsvc:Put_eSignature_Integration_Data_Request[1]/bsvc:eSignature_Signing_URL_Data[1]/bsvc:eSignature_Document_Signing_URL_Data[1]/bsvc:Person_Reference[1]</wd:Xpath>
</wd:Validation_Error>
</wd:Validation_Fault>
</detail>
</SOAP-ENV:Fault>
</SOAP-ENV:Body>
</SOAP-ENV:Envelope>

Would you know what the root cause is of this error and how it can be resolved? 
Thank you in advance.

Kind regards,

Joelle