Mocking Ext.data.StoreManager method call with jasmine

I have the following line of code inside a function I want to unit test:

var myStore = Ext.data.StoreManager.lookup("MyStore");

I am trying to mock the store inside my unit test as follows:

spyOn(Ext, 'data').and.returnValue({
    StoreManager: {
        lookup: function() {
         return myMockStoreObjectForTestingPurposes;
        }   
    }
});

However, for some reason the unit test is trying to use the real objects returned by Ext.data instead of my mock object.

Anyone know why this is happening? Should I be mocking Ext.data.StoreManager in a different way?