Ask HN: NDAs, a warning sign?

I've been approached by someone interested in working together on one of their startup ideas. We haven't discussed it yet, so I have no sense of my actual interest. However, in the past they've required that interested collaborators sign an NDA before discussing the idea – despite it seemingly being something they haven't done anything towards implementing.

I know that Paul Graham has stated "The market price [of a mere idea] is less than the inconvenience of signing an NDA," but that was in regards to the misstep of asking potential investors to sign an NDA before talking with them. What about asking potential collaborators to sign an NDA? Is this a warning sign? I know there are a lot of variables (how well do you know the person, etc) but is this generally something that should inspire caution and potentially refusal?