Finches - dxf invalid files

Hello!

Finches is a great help for working with batch files. 
I've used Finches to import dxf files, process in grasshopper, and then export again as dxf using finches components for import and export. Similar to video tutorials.
But then it happens that I can not open these dxf files in autocad, that program says they are invalid.
Then I have to open each dxf in rhino (rhino can open the files) and then export them as dxf again, one by one (and there are many ...)
Which can be the problem?