chrome.storage.sync undefined?

I'm trying to use chrome storage in an extension, via a content_script, but I keep failing on

Uncaught TypeError: Cannot read property 'sync' of undefined

This is my code:

testChromeStorage();

function testChromeStorage() {  
    console.log("Saving");
    chrome.storage.sync.set({'value': theValue}, function() {
        message('Settings saved');
    });
    chrome.storage.sync.get("value", function (retVal) {
            console.log("Got it? " + retVal.value);
    });
}

You have to add the "storage" permission in your manifest.json file, i.e.:

//...
  "permissions": [
    "storage"
  ],
//...

For more information, see: https://developer.chrome.com/extensions/storage