Declare SignificantDigitsNumberRounder in XAML

[Slion]

Why is it that when I do this:

<f:SignificantDigitsNumberRounder x:Key="Rounder" SignificantDigits="4"/>

I get:

XamlCompiler error WMC0055: Cannot assign text value '4' into property 'SignificantDigits' of type 'UInt32'

I feel like the compiler should be able to do that. Does it means I need to do this in code?