why does my script get the error of Can't Parse JSON when using the HTTP service?

local HTTP = {}
HTTP.URL = "https://raw.githubusercontent.com/PixelLiten/Nethereal/master/0.0.1/files.json"
local http = game:GetService("HttpService")
function HTTP:HTTPGET(url)
    local success, res = pcall(http.RequestAsync, http, {
        Url = url;
        Method = "GET";
    })
    if (not success) then
        res = {
            Success = false;
            StatusCode = -1;
            StatusMessage = res;
            Headers = nil;
            Body = nil;
        }
    end
    if (not res.Success) then
        warn(url .. " >> " .. res.StatusMessage)
    end
    return res, res.Body
end
function HTTP:HTTPGETJSON(url)
    local success, res = pcall(http.RequestAsync, http, {
        Url = url;
        Method = "GET";
    })
    if (success) then
        local json
        success, json = pcall(http.JSONDecode, http, res.Body)
        if (success) then
            res.Body = json
        else
            res = json
        end
    end
    if (not success) then
        res = {
            Success = false;
            StatusCode = -1;
            StatusMessage = res;
            Headers = nil;
            Body = nil;
        }
    end
    if (not res.Success) then
        warn(url .. " >> " .. res.StatusMessage)
    end
    return res, res.Body
end
function HTTP:Install()
    warn "Fetching file list..."
    local filelistRes, filelist = self:HTTPGETJSON(self.URL)
    if (not filelistRes.Success) then
        warn(filelistRes.StatusMessage)
        return
    end
end
return HTTP