How to calculate zakat while in debt?

I recently bought a house and got Shariah loan (Mortgage). The only difference is that in the Shariah loan they don't use the word "interest". They say I am paying them "rent" at a fixed percentage which is same as today's interest rate. Now how do I calculate Zakat on my wealth if I am still in debt? Because if I do so, my net-worth will be negative.