How to enforce double quotes on all template values in input transformer

Question

I have a JSON input that I would like to transform to another JSON output. I defined my list of input JSONPaths, and trying to create a simple JSON output in the template like so:

{
    "fax": \"<userFax>\"
}

This was one of the formats given in the example from AWS themselves:

{
    "instance": \"<instance>\",
    "state": [9, \"<state>\", true],
    "Transformed": "Yes"
}

However, when I try to update the changes, I get the following error:

Invalid InputTemplate for target ... : [Source: (String)"{ "fax": \"null\" }"; line: 2, column: 13].

Basically, I'd like all incoming values in the input to be converted to strings as an output via the template. This is to prevent values like zip codes from being converted into an integer and having it's leading zero stripped away. But it's confusing that even following the simple example from AWS is failing.