sinon a spy function that exists on a controller (this)

I have tricky function that I want to spy on.

 $rootScope.AlertsGlobal = {};

    this.updateHeight = function (clientHeight) {
    $rootScope.AlertsGlobal.clientHeight = clientHeight;

};

This refers to the angular controller

 var GlobalAlertsController = function() {
 }

like so ...

 var GlobalAlertsController = function() {
      this.updateHeight = function (clientHeight) {
    $rootScope.AlertsGlobal.clientHeight = clientHeight;
 }

The function lives inside directives controller and is required by another controller (requires difference in unit test? ).

require: 'MainGlobalAlerts',

Inside of my unit test I mocked out my directive

html = angular.element("<my-directive> </my-directive")
element = $compile(html)($rootScope)
$rootScope.$digest(element)
scope = element.scope()    // note html and scope are defined below describe

I have tried to sinon spy this updateHeight() a couple of different ways.

sinon.spy(scope, 'updateHeight')

sinon.spy($rootScope, 'updateHeight')

However my error statement

 Typeerror: Attempted to wrap undefined property updateHeight as a function

Since updateHeight Is defined with this I am not sure how to call this method if I cannot spy it.