tkk checkbutton appears when loaded up with black box in it

I create a check button / box, with the following call

x=ttk.Checkbutton(tab1,state='disabled',command = lambda j=i,x=k: fCheckButton(j,x)) x.state(['selected'])

The box appears fine and is selected, but it appears on load up, with a black box in it, which seems to have nothing to do with the state of it.

I have looked for reasons why, but can't actually find anyone with the same problem.

thanks