Passing Arguments in XAML

I'm trying to pass arguments in XAML to my own class following:
passing arguments

like:

<MyClass>
    <x:Arguments>
            <x:Boolean>true</x:Boolean>
            <x:Double>90</x:Double>
    </x:Arguments>
</MyClass>

But when I add this I get an error saying:

The given key was not present in the dictionary.

If I hard code it inside my constructor instead, it all works fine, but I want to be able to change it.