What fallacy is this?

[OmegaGuy]

I got into a debate and this came up.

The basic gist of the argument my opponent was using is:

"A has never been subjected to attack B, therefore A has no defense against B"

This seems like a fallacy to me, but I can't identify it.

[Mange]

Hmm, it seems as a non sequitur to me.

[mr friendly guy]

That's like saying a modern day battleship has never been attacked by bows and arrows before, therefore it has no defences against bows and arrows.

This strikes me as a non-sequitar, ie it doesn't follow. However all fallacies can most likely be broken down into non-sequitars.

[Darth Wong]

I got into a debate and this came up.

The basic gist of the argument my opponent was using is:

"A has never been subjected to attack B, therefore A has no defense against B"

This seems like a fallacy to me, but I can't identify it.

Fallacy of unstated premise. The logic works if you assume that B is particularly dangerous to A, so that the burden of proof is upon anyone who would say that A can survive B.

[DaveJB]

I got into a debate and this came up.

The basic gist of the argument my opponent was using is:

"A has never been subjected to attack B, therefore A has no defense against B"

Just point out that there's no more evidence for that position than if you said "B has never been seen to have any effect on A, therefore B is totally ineffective against A."

[Alerik the Fortunate]

In light of the last reply, I would probably say appeal to ignorance as the chief fallacy.