Sverchok - 'int' object is nog iterable

Not sure if this is the right place (my first post), but I suppose it is Python-related.

In Sverchok I have made 4 vertices, combined them with List Join (level 2, no mix or wrap) and feed that into ‘vertices’ of Viewer Draw. To make a face, I use Formula with [[3,2,1,0]] and feed that into ‘edg_pol’. This works fine.
Now, I want to extrude this, so I use Extrude Separate and feed the above into ‘Vertices’ and ‘Polygons’ and feed the outputs with the same name into Viewer Draw. This turns the Extrude node red, and in the console I get “Node Extrude Separate.001 had exception ‘int’ object is nog iterable”.