random – 函数NameGen()：尝试将nil与数字进行比较

[string "function NameGen()..."]:14: attempt to compare nil with number
stack traceback:
    [string "function NameGen()..."]:14: in function 'NameGen'
    [string "function NameGen()..."]:23: in main chunk
    [C]: ?

我的代码：

function NameGen()
  preftest = math.random(100) ;
  syltest = math.random(100) ;
  sufftest = math.random(100) ;
  pref1 = "New ";
  _1syl1 = "Lon";
  _2syl1 = "Don";
  suff1 = " City";
  prefchoice = pref1;
  _1sylchoice = _1syl1;
  _2sylchoice = _2syl;
  suffchoice = suff1;

  if preftest < 50 and _2syltest < 50 and sufftest < 50 then 
    cityname = prefchoice .. _1sylchoice .. _2sylchoice .. suffchoice;
  elseif preftest < 50 and _2syltest < 50 then
    cityname = prefchoice .. _1sylchoice .. _2sylchoice;
  else
    cityname = _1sylchoice;
  end
end
NameGen();
print(cityname);