Xcode 4.2 Warnings when dropping Nav Controller on Tab Bar in IB

I'm developing an app which is iOS 4 compatible, so my deployment target is set to iOS 4.0.

Whenever I drop a UINavigationController onto a UITabBar, I get these two warnings:

1. warning: Attribute Unavailable: Defines Presentation Context is not available prior to Xcode 4.2.
2. warning: Attribute Unavailable: Defines Presentation Context on iOS versions prior to 5.0.

The UINavigationController functions as expected, in fact, the entire app runs perfectly. But these two warnings are driving me nuts!

Also, the moment I delete the UINavigationController the warnings disappear.