Lua Coroutines resume : bad argument #-1 to 'resume' (expected bool)

Keywords´╝Ü lua


I'm kind of very new to lua and just inherited a lua code base where I see an error happening occasionally on resuming a coroutines

--coroutine creation
self._active_test_worker =
            return self._active_test:worker(timestep)

--coroutine resume 
local temp = type(self._active_test_worker) //thread
local temp2 = type(timestep) // number
local statsu = coroutine.status(self._active_test_worker) // suspended
local success,action = coroutine.resume(self._active_test_worker, timestep)

The 'resume' function raises the following error intermittently

factory.lua:478: bad argument #-1 to 'resume' (expected bool)

Can someone please help me in understanding the error? What is argument -1? ( what does negative mean here?) What might raise this error? I couldn't find much online or in the docs about this error. Any help would be welcome.