Why does Lua 5.4.3 not like LuaSocket 3.0-rc1 default receive() call?

[Paul K <paul@...>]

It's been reported that luasocket receive() call fails on Lua 5.4.3
with "bad argument #1 to 'receive' (string expected, got light
userdata)" (see https://github.com/diegonehab/luasocket/issues/331).
The earlier versions of Lua don't throw this error.

While the fix is simple (use `receive("*l")` instead of `receive()`),
does anyone know what triggers this particular error in luasocket?
Thanks.

Paul.