How can I edit a function from a package which is invoking UseMethod?

Question

I need to make some changes in vcovG function of the plm package. However, when I call trace(vcovG, edit = T, where = plm), I get the following in the editor:

function (x, ...) 
{
  UseMethod("vcovG")
}

In the source code, the function is defined by the name vcovG.plm. But I call trace(vcovG.plm, edit = T, where = plm), I get an empty function in the edit window. Any ideas where I am going wrong? Thanks in advance.