Weird error on Reverse a String

JavaScript
1

I tried a solution to this challenge using recursion:

function reverseString(str) {
    let lastIndex = str.length-1;
    if (lastIndex === 0) return str;
    return str[lastIndex] += reverseString(str.slice(0, lastIndex));
}
 
reverseString("hello");

It works in the developer console in more than one browser I have, but freeCodeCamp just says “1 is read-only”. Is this a bug with freeCodeCamp?

Firefox 61.0.1 on Windows,
Microsoft Edge 42.17134.1.0 on Windows

Link to challenge