Slit conditions error

Hi Everyone.

I am working on Dielectric Resonator Antenna in Comsol 4.4. When I complete my design and try to compute, it always has an error.

"The slit condition should be applied only on an interior boundary or the port should be placed on an exterior boundary."

What is this slit conditions in Comsol.? Can anyone help me how to solve this problem. Its urgent. Thanks in advance.