Adding an Element to a sub node

I have a Context that consists of a node (foodAndBevServingSizeInfo), three additional attributes, and a sub node (householdServingSize) which has a single attribute. Please see below image.

My problem is I struggle to set the value of the "value" attribute of the "householdServingSize" sub node.

Can you please guide me on how to achive it?

My code below does not seem to work:

I'm getting this error:

while trying to invoke the method com.sap.tc.webdynpro.progmodel.context.Node.addElement(com.sap.tc.webdynpro.progmodel.api.IWDNodeElement) of an object returned from com.sap.is.gds.custom.interspec.web.wd.wdp.IPublicInterspecWebComponent$IFoodAndBevServingSizeInfoElement.node(java.lang.String)

Thanks in advance

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