Ok, it turns out I wasn't actually passing the values i stated i passed to set_signal(). i had some construction like:

behavior.set_signal(n, {signal=signal, count=c})

and i assumed signal set earlier would be passed to set_signal() as is.
set_signal(n,null) works as expected. Thanks for helping ...

How do I unset constant combinator signal? I tried using LuaConstantCombinatorControlBehavior.set_signal like this:

behavior.set_signal(n, nil)
behavior.set_signal(n, {})
behavior.set_signal(n, {type="item"})
behavior.set_signal(n, {type="item", name=""})

None of the above work, they all throw ...