Why does Mathcad return an equation including i instead of a numerical value.

Hi all. I have a model that calculates heat transfer coefficient. It is supposed to and does most of the time, return a numerical value (an integer) for the heat transfer coefficient. However, sometimes it will return a complex number, e.g. 20 + 5i, instead of an integer. What does i stand for in Mathcad and how can I get the model to return an integer?