Declaring foreign rental property on tax return for 2011

I have a mortgaged property in another EU country (which has a double taxation agreement with Belgium) which has been rented out since August 2011. It seems I have to declare this property in Belgium but I cannot give an accurate income amount, although it will certainly be a minus figure for 2011. The tax form in my own country is not due until August this year and I want the tax inspectors there (or an accountant) to calculate my liability properly(if any). So, what should I say on the Belgian tax return? Many thanks for any guidance.