The 'clr-namespace' URI refers to a namespace that is not included in the assembly

I'm trying to include in my XAML some classes which convert values. However, I'm getting the following error when I compile:

Undefined CLR namespace. The 'clr-namespace' URI refers to a namespace 'View.Summary.Converters' that is not included in the assembly.(View\View)

And the XAML it's erroring on:

xmlns:c="clr-namespace:View.Summary.Converters"

Also, here is the outline of my conversion classes/namespace:

namespace View.Summary.Converters
{
    class CollapsedIfNegative : IValueConverter { }

    class VisibleIfNegative : IValueConverter { }

    class ErrorCodeToString : IValueConverter { }
}

I had to remove the guts of the code because the project I am working on is highly confidential.