qgis - Why doesn't Polygonizer copy line ids to polygonized shapefile?

I am trying to convert a line shapefile into a polygon shapefile using Polygonizer plugin, and when my new polygons are created all the cells in my id field are "Null", although I have clicked "copy table field from line layer". I have tried both Polygonizer 2 and Polygonizer 2.1.

Any idea why the ids from my line layer are not copied to my polygonized shapefile?