WoW Lua错误; 尝试调用全局变量 (a nil value)

我是Lua的新手，正在遵循_https://wowwiki.fandom.com/wiki/AddOn_tutorial_ 上的教程，但我无法使它正常工作; 我已经复制了代码，但我在**SetMapToCurrentZone ()**和 GetPlayerMapPosition("player") 函数中都收到了错误消息 "尝试调用全局'函数名' (a nil value)"。

这是整个Lua文件;

local zone = nil
local TimeSinceLastUpdate = 0
local function UpdateCoordinates(self, elapsed)
  if zone ~= GetRealZoneText() then
    zone = GetRealZoneText()
    SetMapToCurrentZone()
  end
  TimeSinceLastUpdate = TimeSinceLastUpdate + elapsed
  if TimeSinceLastUpdate > .5 then
    TimeSinceLastUpdate = 0
    local posX, posY = GetPlayerMapPosition("player");
    local x = math.floor(posX * 10000)/100
    local y = math.floor(posY*10000)/100
    eCoordinatesFontString:SetText("|c98FB98ff("..x..", "..y..")")
  end
end

function eCoordinates_OnLoad(self, event,...)
  self:RegisterEvent("ADDON_LOADED")
end
function eCoordinates_OnEvent(self, event, ...)
  if event == "ADDON_LOADED" and ... == "eCoordinates" then
    self:UnregisterEvent("ADDON_LOADED")
    eCoordinates:SetSize(100, 50)
    eCoordinates:SetPoint("TOP", "Minimap", "BOTTOM", 5, -5)
    eCoordinates:SetScript("OnUpdate", UpdateCoordinates)
    local coordsFont = eCoordinates:CreateFontString("eCoordinatesFontString", "ARTWORK", "GameFontNormal")
    coordsFont:SetPoint("CENTER", "eCoordinates", "CENTER", 0, 0)
    coordsFont:Show()
    eCoordinates:Show()
  end
end

我该如何解决它？