I created a woven closed loop belt (for nuclear reactor fuel feeding/removal) and it gives me the error below whenever I mouse-over an underground belt exit in either loop. It doesn't seem to cause any issues other than giving the error on mouse-over and it doesn't show the full path loop (the yellow overlay) when on the exit. It shows the full path overlay when I mouse-over any other segment, but only the underground portion when I'm on an underground exit.

PickerBeltTools/scripts/belt-highlight.lua:418: attempt to get length of local 'input' (a nil value)

Can you post a screenshot of the setup?

https://imgur.com/Yjfvoxm
https://imgur.com/7JvsHPu

In the second my cursor is on the right end of the highlighted underground belt, so the "entrance".

have the same error for a belt my self. im running a heavily modded game and thought i'd see just how far i could go with a spaghetti Mall set up... it took a wile to track down.
https://steamuserimages-a.akamaihd.net/ugc/789740278445542588/995DA1465BC5F1A98813B9CBB37FAB2CEBC1F597/

tracked it down to this highlighted spliter. anything upwind and connected to it returns the nill value error.
down wind is fine and goes through the spliter fine.

https://steamuserimages-a.akamaihd.net/ugc/789740278445542805/477A0E45B641CDA491932DCBA4562C0D3BF5B775/
as for as i can tell the belt doesn't loop at any point. though it may weave at one point or more... im not entirely sure.
https://steamuserimages-a.akamaihd.net/ugc/789740278445543076/6D1BB3B36175157AD7EF227EE610F56A080AF5E7/

and a full zoom out so you can see the rest of the path.

sry for the necro

Not a Necro. Just an awesome use of beltmarker. Thanks I'll replicate it and figure out what's up. I have a feeling I can tell what it is but I have to check it out.

I've opened a PR that should fix the issue: https://github.com/Nexela/PickerBeltTools/pull/20