You must assume either that both tree and fruit are good, or that both tree and fruit are worthless; since it is by it's fruits that a tree is known.

And, noticing a solitary fig tree by the road-side, he went up to it, but found nothing on it but leaves. So he said to it: "Never again shall fruit be gathered off you." And suddenly the fruit tree withered up.

So, too, every sound tree bears good fruit, while a worthless tree bears bad fruit.

A sound tree cannot produce bad fruit, nor can a worthless tree bear good fruit.