NullReferenceException with IE but not FF?

This is the first time I've seen the compiler complain with IE [1] but not
Firefox. So how do I write code that apparently finds a control and is used
logically with one browser (Firefox) but then fails with
System.NullReferenceException when using IE?

// IE doesn't like this
Button fb = Page.Master
.FindControl("CenterPanelContent")
.FindControl("ChannelBuilderWizard")
.FindControl("StepNavigationTemplateContainerID")
.FindControl("FinishButton") as Button;

[1] Object reference not set to an instance of an object.