Handling XML using xsi:nil attribute (IS 7.1.2)

I’ve got a problem with proper handling of XML with xsi:nil attribute. I have document which is constructed in flow service. Attributes are defined using “@attributeName” method - see attachment example.

Then I call service pub.xml:documentToXMLString and everything is fine, XML as string is produced with attribute xsi:nil=“true”. Then pub.xml:xmlStringToXMLNode service is called, without problems. Then service pub.soap.utils:addBodyEntry is used to put it into SOAP envelope - error!
pub.soap.utils:addBodyEntry [ISS.0088.9131] Invalid node: must be well-formed XML

I know it’s because of xsi:nil attribute. But how to get it working?