Can't allocate Non-Maryland Income for Sale of Rental House

I am a resident of WA and sold a rental house in MD. I reported the sale in Federal and have a capital gain due to depreciation. However- in the MD nonresident return, that capital gain is not being allocated to MD. There is a page called Non-Maryland Income Allocation which lists the Federal total and the Non-Maryland total for capital gains, rents, etc. It says, "In the right column, enter amounts that should not be included in your Maryland taxable income".  But, the right column is greyed out  (not editable) and I can't allocate the capital gain to Maryland. How can I report it?