https://developer.roblox.com/articles/JSON-Storage-Format
JSONEncode cannot encode anything other than strings, numbers, tables, and booleans - passing anything else to it will error.
print(game:GetService'HttpService':JSONEncode{CFrame.new()}) -- [null]
print(game:GetService'HttpService':JSONEncode{newproxy()}) -- [null]
It doesn’t error