Must "attempt to get length of a number value" be an error?

[Dirk Laurie <dirk.laurie@...>]

Lua 5.3.5  Copyright (C) 1994-2018 Lua.org, PUC-Rio
> #-5
stdin:1: attempt to get length of a number value
stack traceback:
    stdin:1: in main chunk
    [C]: in ?
> debug.setmetatable(0,{__len=math.abs})
0
> #-5
5