How to fix error, "Argument 1 missing or nil", when I am making a DataStore?

I am currently trying to make a DataStore, and error, " 19:10:00.118 - Argument 1 missing or nil", shows up. Here is the piece of code that it is showing up on,

local function InitializeSync()                                                                         

    local NewData = game:GetService("HttpService"):JSONEncode(Sync.Data);
     local OldData = game:GetService("HttpService"):JSONEncode(DataStore:GetAsync("SyncData")); -- this is the line the error shows up on

I would appreciate help, Thanks!