Error format xml in parameters of Call-Activity

When add parameters to a component Call-Activity from the graphical editor, in the file *.bpmn20.xml are not included within a node <extensionElements>.

For example:

<callActivity id="callactivity1" name="Llamada al Subproceso" calledElement="subproceso">
<activiti:in sourceExpression="$

{bpm_groupAssignee}" target="bpm_groupAssignee"></activiti:in>
<activiti:in sourceExpression="${bpm_package}" target="bpm_package"></activiti:in>
</callActivity>


And it should be...

<callActivity id="callactivity1" name="Llamada al Subproceso" calledElement="subproceso">
<extensionElements>
<activiti:in sourceExpression="${bpm_groupAssignee}

" target="bpm_groupAssignee"></activiti:in>
<activiti:in sourceExpression="$

{bpm_package}

" target="bpm_package"></activiti:in>
</extensionElements>
</callActivity>