Atomac can find the window but not the sheet #153

I did follow the instructions on the readme, although I am stuck at the section where you retrieve the sheet.

I can get the window, and the title fine; but when I call window.sheets() I get back an empty list [].
I am on OSX 10.11; is there something that I am missing? I did try with Automator, as in the readme example, and with other apps but it always get stuck when I try to retrieve the sheet from the window. Thanks