Is there a word that means "an argument/proposition potentially vulnerable to attack"?

A friend made an argument that I thought was fairly strong but had fairly easy lines of attack available against it. (Whether or not these lines of attack are strong/valid is unknown.) Is there a good word or phrase that would describe this proposition of his? The best phrases I can think of are "potentially assailable proposition" or "an argument that may have holes in it" or "an argument potentially vulnerable to attack".

Assuming Bob is my friend who just made the argument described above, an example usage would be in response to him:

Bob, I think that is a ____.

Edit: After thinking about this more, a key point in this question is that the argument is only potentially weak but not certainly weak.

Solution 1:

Since you don't dispute the overall argument but consider that there may be some loopholes that could derail it, you may be better off going with the negative form: "Bob, I think this is not a watertight argument."

Watertight 2. (Of an argument or account) unable to be disputed or questioned - ODO

Solution 2:

You could consider using fallacy which means:

A fallacy is an incorrect argument in logic and rhetoric which undermines an argument's logical validity or more generally an argument's logical soundness. Fallacies are either formal fallacies or informal fallacies.

There are many types of fallacies and you could see them in the link.

[Wiktionary]

Solution 3:

An argument intended to promote a serious discussion could be called a 'tentative argument'.

Otherwise, it doesn't hold water, which implies the burden of proof has not been met.