Fail while building app for iOS

Hi, I have a little project so I could get known with Xamarin.Forms. It is an app using only Xamarin.Forms elements and xaml for views. I also use Azure.
The app works on Android and WinPhone, but when I try to build it on my build server for iOS I get this exception:
System.TypeInitializationException: An exception was thrown by the type initializer for ShoppingList.App, and it stops.

Any idea why?