Hyphens, En Dashes, Em Dashes

Q. I’m taking a popular online copyediting course. One of my answers to a quiz was marked wrong because I failed to identify “early-warning system” as an instance of incorrect hyphen usage. According to the answer key, this is incorrect because adverbs ending in -ly should not be followed with hyphens. I think early is used as an adjective in this example and should therefore take a hyphen.

A. You’re right. A hyphen after early may also be needed to prevent ambiguity: early voting statistics aren’t necessarily early-voting statistics.