jQuery $.parseJSON(); behaving differently in two calls

I’m using jQuery to get and parse some JSON from my ASP.Net MVC backend. I’m recieving the data correctly:

data: "{"0":"Imgs/staticResistor.png","1":"Imgs/wire.png"}"

Function to get data:

$.getJSON('https://localhost:44338/Editor/GetImgUrlFromTypeID/')
                .done((data) => {
                    console.log("JSON: " + $.parseJSON(data)[0]);
                    let json = $.parseJSON(data)[0];
                    //return $.parseJSON(data);
                })
                .fail((error) => {
                    console.log("GET request failed: " + error);
                    return "didnt work";
                });

However, when I assign $.parseJSON(data)[0]; to a variable or return it, it’s undefined:

json: undefined

I can’t discern why the method is behaving differently in different calls.