Unknown DataStore error?

I am using the UpdateAsync function to save purchase history and I am getting the error 'string is not allowed in DataStore'

Key is 'userId'

Transform Function

Value returned from transform is '{ProductId:[{"Awarded":true,"Time":Time}]}' (Not literally, I've replaced ProductId and Time accordingly)

Does anybody have any idea what this error implies?

Note: I have created my own global environment, most functions (such as 'DataStore:Update()') are just proxies. You do not need to know what any other function does.