Ok, on trial & error , this is what I noticed. Even though the income is zero, on the interview page for the spouse, I clicked both the options , 1st - No income & the 2nd - T4 / T4A with the thought I will put 0 on the T4.
Just by clicking both, the "control option" for spouse opens up & you get the drop down to select 'Nil income' . Hope this is correct & helps.
Mohan