Xaml File does not see Class from a Namespace

Question

Why does the App.xml not find the Class CaptionButtonRectToMarginConverterwithin the Shell \ CaptionButtonRectToMarginConverter.cs File ?

Error XDG0008 File App.xml Line 13

The name "CaptionButtonRectToMarginConverter" does not exist in the namespace
"clr-namespace:ErdBuilder.Shell".

App.xml

<nRoute:Application x:Class="ErdBuilder.App"
                    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:nRoute="http://nRoute/schemas/2010/xaml"
                    xmlns:chronos="http://chronos/schemas/2010/xaml"
                    xmlns:shell="clr-namespace:ErdBuilder.Shell"
                    StartupUri="Shell\Shell.xaml"
                    ShutdownMode="OnMainWindowClose">

    <nRoute:Application.Resources>
        <ResourceDictionary>

            <shell:CaptionButtonRectToMarginConverter x:Key="CaptionButtonMarginConverter" />

Shell \ CaptionButtonRectToMarginConverter.cs

namespace ErdBuilder.Shell
{
    public class CaptionButtonRectToMarginConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            ...
        }
    }
}