"Open-mindedness" means nothing. Stop saying it to dismiss people for being confident in sound reasoning.
The problem is that asking someone a question like "Are you willing to change your beliefs if they're proven wrong?" in the context of a belief that is held to be universal and necessary (like a claim about metaphysics) is wrong because it requires them to consider a scenario that is, from their perspective, logically impossible.
But either answer damages their image. If they say no, they're seen as willfully irrational because they're outright saying they'd reject evidence if it didn't mesh with their preconceived beliefs. If they say yes, they're saying that such a thing is possible, and so they're saying they're not certain of their beliefs. With your appearance of confidence destroyed your chance of winning an argument is crippled. Worse, in the case of rejecting materialism, that's an automatic defeat because you'd need to have absolutely reliable a priori reasoning to reject entire fields of science and the consensus of everyone else on the planet and allegedly thousands of experiements.
I've even fallen for this somewhat recently. I was asked what I would make of it if a neurologist happened to predict my thoughts accurately, and I said it would have to be a coincidence. Utter failure. The coincidence was built in to his question and not my answer. But I failed to understand the trick being played on me. Luckily, the results were contained since I was only with two people that will never be swayed anyway no matter what, so it's a good thing in the end. I learned something about dirty argument tactics without a disastrous real life result.
What I wish I had said in retrospect is, "The same thing you'd make of it if I proved that you don't exist."