Lua错误PANIC：在调用Lua API时出现未受保护的错误（尝试调用空值）。

这是一段代码，在执行时会报错："PANIC: unprotected error in call to Lua API (attempt to call a nil value)"

#include <stdio.h>
extern "C"{
#include "lua.h"
#include "lualib.h"
#include "lauxlib.h"
};

lua_State *L;
int luaAdd(int x, int y)
{
int sum;
lua_getglobal(L, "add");
lua_pushnumber(L, x);
lua_pushnumber(L, y);
lua_call(L, 2, 1);
sum = (int)lua_tonumber(L, -1);
lua_pop(L, 1);
return sum;
}

int main(int argc, char *argv[])
{
  int sum = 0;
  L = lua_open();
  luaL_openlibs(L);
  luaL_dofile(L, "add.lua");
  sum = luaAdd(10, 15);
  printf("The sum is %d\n", sum);
  lua_close(L);

  return 0;
}

add.lua

function add(x, y) do
  return x + y
end
end

能告诉我哪里错了吗，谢谢。