On page 1 of T1, there is a field for spouse net/taxable income. Due to space limitation, only the spouse net income is displaying. When you click mouse into the field, or try to type a number into that field, a supporting field window will pop up, like the following:
There are both spouse net income and taxable income in this window. Make sure both spouse income fields are entered, and taxable income should not be more than net income.