Tooltip breaks in Internet Explorer unless tooltip directive references an object

[crackernutter]

works in IE and Chrome.

However, only works in Chrome, and generates the following error for IE:

"Error in directive tooltip update hook: "TypeError: Object.keys: argument is not an Object"

Is this expected or a known error? Do I have to change all my tooltip directives to objects for it to work in IE (IE 11.0.9600 currently).