System.MethodAccessException: Method `Xamarin.Forms.LayoutOptionsConverter..ctor()' is inaccessible

I am brand new to Xamarin development, and so far I can't get any of the projects to work out of the box.

I created a new ContentPage XAML page, and then assigned it to the MainPage instance in my App:

public App ()
{
    // The root page of your application
    MainPage = new Page1();
}

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="<redacted because I can't post links>"
             xmlns:x="<redacted because I can't post links>"
             x:Class="Namespace.Page1">
    <ContentPage.Content>
        <StackLayout>
            <Label Text="HELLO WORLD"
                VerticalOptions="Center" 
                HorizontalOptions="Center" />
        </StackLayout>
    </ContentPage.Content>
</ContentPage>

The exception is coming from the InitializeComponent method of the Page1 class's constructor.

Any ideas? A Google search doesn't yield any specifics.