Hello Kellie,
To workaround the problem you should use the full XML namespace for this converter (the other elements should be OK with the URI namespace):
xmlns:telerikConverter="clr-namespace:Telerik.Windows.Controls;assembly=Telerik.Windows.Controls"
<telerik:RadMenuItem.IsChecked>
<Binding Path="IsPinned">
<Binding.Converter>
<telerikConverter:InvertedBooleanConverter/>
</Binding.Converter>
</Binding>
< /telerik:RadMenuItem.IsChecked>
Greetings,
Valeri Hristov
the Telerik team
Explore the entire Telerik portfolio by downloading the Ultimate Collection trial package. Get it now >>