CST - warning: accepted power is very small, gain is inaccurate

Hello,

I am trying to simulate a d​​​​​​​ual-feed antenna in CST with simultaneous excitation in the two ports. Although the F-parameters as well as the S-parameters show good isolation and low return loss; but one of the farfield patterns always says "warning: accepted power is very small, gain is inaccurate." Could you please explain this? Thank you.

CST Studio Suite ​​​​​​​