ComboBox not selecting null

I have a FormFieldFactory that creates a ComboBox:

private ComboBox callClassifications;
callClassifications = new ComboBox("Call Classifications");
callClassifications.setRequiredError("Call Classification cannot be empty!");

I have a setObject function:

public void setObject(CallRoute cr)
	if (cr == null)
		cr = new CallRoute();
	} = cr;
	this.crItem = new BeanItem(cr);
	f.setVisibleItemProperties(Arrays.asList(new String[]
			"callClassification_id", "portIds"

If I set the CallRoute to an instantiated object, the CallClassification ComboBox is filled in correctly. After the edit (or add) is done, and the form is reset with my setObject function, the value in the ComboBox stays the same as the last object. (Even though the constructor in CallRoute sets it to 0).

How would I make the value of the ComboBox get reset to null?