Aaah, yeah, sometimes it’s wise to check that our inputs are as expected! ^^;
There is a convenience function/technique that allows one to pick a point on a plane–in your case you would simply set up the plane to align with the Y-Z plane. See here:
https://discourse.panda3d.org/t/super-fast-mouse-ray-collisions-with-ground-plane/
(It references a “ground plane”, but it should work for any arbitrary plane, I believe.)