API调用和CORS政策错误：我错过了什么？

Question

我正在尝试对打开的Web服务器进行简单的API调用。这是我实现此目的的代码：

function main()
{

    makeAPIcall_GET(
        'https://icanhazdadjoke.com/', // URL

        function(response) // callback function
        {   
            console.log(response);

            try{
                var parsedResponse = JSON.parse(response);
            }catch(e){
                logError(e);
            }
        }
    );
}


function makeAPIcall_GET(url, callback)
{
    var xhr = new XMLHttpRequest();
    xhr.open("GET", url);

    xhr.setRequestHeader("Content-type", "application/json; charset=utf-8");
    xhr.setRequestHeader("Accept", "application/json");

    xhr.send();

    // handle success
    xhr.onload = function(){
        callback(xhr.responseText);
    }
}

这是我收到的CORS政策错误：

Access to XMLHttpRequest at 'https://icanhazdadjoke.com/' from origin 'null' 
has been blocked by CORS policy: Request header field content-type is not allowed by 
Access-Control-Allow-Headers in preflight response.

我浏览了Internet，希望找到解决方案，但是我不得不说我不明白为什么这行不通！谢谢。