Can't allocate mouse wheel for any action after switching the pick block function to another key

A long time ago, i allocated the R-Key for the Pick-Block-function.
Now i want to switch it back to the mouse wheel, which doesn't work.

What I expected to happen was...:
I'm able to allocate the mouse wheel to the Pick-Block-function.

What actually happened was...:
It didn't work.

Steps to Reproduce:
1. Allocate any key for the Pick-Block-Function
2. Try to reallocate the mouse wheel.