Milan had Robinho's shot cleared when the ball had possibly crossed over.
"When it crosses the line, the rules say that it is a goal, but not for us," rapped Milan vice-president Silvio Berlusconi.
"It is the second time this has happened to us. I understand refereeing errors can happen and we accept them, but I really don't understand why our goals that cross the line are not given. It's frankly unacceptable.
"In other games the officials have been very good at spotting these incidents and getting the right call. It looks as if we are just very unlucky.
"I hope this bad luck ends and that this incident does not alter the Scudetto race."