Can I (reasonably) refuse to sign an NDA for pro bono work? [closed]

Question

A friend of mine (let's call him Joe) is working on a promising project, and has asked me for help. As a matter of friendship I agreed (orally) not to discuss the details, but now he has a potential investor who wants me to sign a non-disclosure agreement (NDA). Since thus far all my work has been pro bono I told Joe I am not comfortable putting myself under documented legal obligation without some kind of compensation for the risk. (It needn't be strictly financial. I would accept a small ownership stake or possibly even just guaranteed credits in the code and documentation.)

Is my request reasonable, or am I just introducing unnecessary complexity?