How to handle with return number given by lua_resume?

Question

I have some coroutine handling code like this:

int coro_re_num = 0;
int coro_state = lua_resume( coro_lua, master_lua, narg, &coro_re_num);

if ( coro_state == LUA_OK)
{
    // do something
}
else if ( coro_state == LUA_YIELD)
{
    // do_something
}
else
{
    const char* err = lua_tostring(coro_lua, -1);
    log_error(err);
}

lua_pop(master_lua, coro_re_num);

When coroutine ends with OK or YIELD, it works properly. But when it ends with any error, the last lua_pop always fail, and I noticed the coro_re_num is relatively large (mostly more than 10) which causes stack underflow by lua_pop.

What does the coro_re_num means when the coroutine ends with error? How to handle it?