Thanks for your reply!
That clears it up. I was actually confused because earlier I was getting the same message "can't evaluate at points...," but for another reason: the points appeared to be in the domain, but were not in the domain of the actual called function. The function was (1-3tanx)^(1/3), and I was getting the same message because it was evaluating where the base was negative. It's my understanding that this is because it was really computing e^[(1/3)ln(1-3tanx)].
Are there any other situations like this - where the actual called function has a different domain from the one used in the problem - that could cause such error messages?