Move group(s) into a group

I want to write a function to grouping selected layers and groups into a new group.

It works fine with layers, but with groups I get an error "Illegal argument".

In a document with just two groups (and the background layer) this code doesn't works and I dont know why :S

// I want move "Group 1" into "Group 2"
var g1 = app.activeDocument.layers.getByName("Group 1");
var g2 = app.activeDocument.layers.getByName("Group 2");
g1.move(g2, ElementPlacement.INSIDE);

Thanks in advanced