"cannot Perform action, System Protected against such attacks"

Ok so I'm playing a game of SEIV with 4 other computer players. i wiped out 3 of them in the beginning. the other race left is the Eee consortium, which atm is much more powerful than me. i decide to use our partnership treaty to sneak in some ships that are capable of turning his stars into blackholes, as soon as i have everything in place, i start blowing up his stars. but when i get to some of his systems( one that i believe is his home system) it told me that it couldnt do it cause that system was protected against that attack. i tried to find a structure or a module that could prevent it but i couldent... anyone know what went wrong? or did i just not look hard enough?