Select random object from JSON

I have the following code:

$.getJSON('js/questions1.json').done(function(data){
        window.questionnaire = data;
        console.log(window.questionnaire);
        startGame();
    });

This brings a json from the server and logs it into a variable. Now after this, I want to select a random question located in the questions.json document:

function pickRandomQuestion(){
        window.selectedquestion = window.questionnaire[Math.floor(Math.random * window.questionnaire.length)];
        console.log(window.selectedquestion);
        console.log(window.questionnaire);
    }

However, when console.log() the selectedquestion variable, nothing comes back, it's undefined. Is there something wrong with my code? I've tripled checked it and I see nothing bad on it, but it might just be my head playing games with me.

Here's how the json looks:

"q1" : {
        "question" : "This country is one of the largest wine-producing countries of the world, where wine is grown in every region of the country. Which country is this?",
        "a"        : "France",
        "b"        : "Italy",
        "c"        : "Germany",
        "d"        : "Australia",
        "corrrect" : "b"
    },
    "q2" : {
        "question" : "What is the name for the type of art portrait that deliberately exaggerates a person?",
        "a"        : "Environmental",
        "b"        : "Cartooning",
        "c"        : "Caricature",
        "d"        : "Tribal",
        "corrrect" : "c"
    },
    "q3" : {
        "question" : "Who was the first president of the United States?",
        "a"        : "Abraham Lincoln",
        "b"        : "Ronald Reagan",
        "c"        : "George Washington",
        "d"        : "Barack Obama",
        "corrrect" : "c"
    }...