Why might Spline from Points only return Length 0?

I’m trying to figure out how to use Spline from Points, but I don’t understand what I could be doing wrong.

I’m starting with just two inputs: Start, and End. If I add two points to the spline, and set 500 x in Start, and -500 x in End, the Length is still 0.

Is there some reason this might be happening? I’ve removed every factor except for the spline itself (so my whole thing looks like: [Spline from Points, Start = 500, 0, 0, End = -500, 0, 0] → [Get Length of Spline] → Print [Length of Spline]), and it’s still doing this.

Edit for further detail:
I can run the Spline Through 3 Points node, and it works. Which is especially confusing, because if I look at how the ST3P works, it uses the Spline From Points node. Which is not working for me.