Question on Subtransform in DTL to transform from a JSON/XML to a custom message

I am  building a DTL to transform from a JSON/XML to a custom message. The DTL works fine, but I wants to move some part of the data transformation to another DTL and calls it using the <subtransform> action from the main DTL.

to define 

I have the  <subtransform class='ADT.DTL.SDAToPatientIdentifierInfo' targetObj='target.PatientIdentifierInfo.()' sourceObj='source.{Patient.Extension.PatientIdentifiers(k1)}' />

when I am testing this subtransform I am getting the following for each identifier in the message.

<PatientIdentifierInfo>
    <PatientIdentifier xsi:nil="true" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"/>
    <PatientIdentifier xsi:nil="true" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"/>
    <PatientIdentifier xsi:nil="true" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"/>
    <PatientIdentifier xsi:nil="true" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"/>
    <PatientIdentifier xsi:nil="true" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"/>
    <PatientIdentifier xsi:nil="true" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"/>
    <PatientIdentifier xsi:nil="true" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"/>
    <PatientIdentifier xsi:nil="true" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"/>
    <PatientIdentifier xsi:nil="true" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"/>
    <PatientIdentifier xsi:nil="true" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"/>
    <PatientIdentifier xsi:nil="true" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"/>
  </PatientIdentifierInfo>

My expectation is receiving the PatientIdentifierInfo in array of JSON format.

Does any one knows why?