Why am I getting a json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

My very simple flask application:

@app.route("/login",methods=["POST"])
def login():
    
    if request.method=="POST":
        username=request.form["username"]
        if not username:
            return redirect(url_for("index"))
        else:
            url="https://www.instagram.com/"+username+"/?__a=1"
            response=requests.get(url, headers={ 'User-Agent':request.headers.get("User-Agent"),'Content-Type':'application/json; charset=utf-8'})
            json=response.json()
            img=json["graphql"]["user"]["profile_pic_url_hd"]
            
            log_user(username)
            return render_template("login.html",username=username,img=img)
            
    return render_template("login.html",username=username)

But I don't understand ** json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0) ** I get this error.

Actually the code works fine on localhost but when I deploy to Heroku I get an error.

Please help me.