Try to put it in an document variable instead of accessing the property of the Document Form. The way you do it should be fine, but for some reason LC might not like it.
Just use a setValue before and extract the document in a document variable and see if that works.
I tried to put /process_data/NameForm in a document variable (with a setValue) and send it, but exception.
I tried to put /process_data/NameForm/object/@document in a document variable, with a setValue, and, after, export this document variable(with exportData) in an other document variable and send it, but exception.