Upgraded to 1.3, iOS app won't compile (CS0234 on FormsApplicationDelegate base class)

I have followed the instructions for upgrading an existing XF app to 1.3, but I cannot get the iOS project to compile. When I change my AppDelegate's base class to FormsApplicationDelegate, I get the below compiler error:

Error CS0234: The type or namespace name Platform' does not exist in the namespaceXamarin.Forms'. Are you missing an assembly reference? (CS0234)

I have the Xamarin.iOS assembly in my References, as well as the XF package's references.