Yes, but with a full function in front of me I got the message.
Because I assumed the nearest point would return the nearest vertex of a line.
Look at the code, it uses a Geometry.Line3d instance which has none vertex: it is unbounded.
The Geometry namespace contains plenty of classes (such as points, lines, segments, circular arcs, vectors, matrices, and so on) which aren't DBObject and are provided to solve geometry issues.