insertColumn with text in 2 header rows

I am wanting to use insertColumn() to add a column to the grid. However, I have 2 header rows that need text in them. The insertColumn() only shows how to add text to the first header row. How can I get custom text in to the 2nd header row on a new column?

I am trying to populate the newly created column 2nd row header with a custom counter. The newly created column index is stored in the variable colIndex. I’ve tried:

mygrid.setColLabel(colIndex, "{#custom_sum}", 2); 

Firebug returns this error:
g is undefined dhtmlxgrid.js (line 69)

I also tried using accessing through the DOM with:

mygrid.hdr.rows[2].cells[colIndex].innerHTML = "{#custom_sum}"; 

Firebug returns this error:
mygrid4.hdr.rows[2].cells[colIndex] is undefined

Any suggestions on what I am doing wrong or should I be approaching this problem another way?

I was able to find my error, I was passing the column id, not the column index.