I have a WPF app that references another project (C# DLL). The WPF project won't compile. Getting:

The type reference cannot find a public type named 'RuleGroup'. Line 12 Position 70.	

Here's the XAML:

<UserControl x:Class="RulesEngineUI.crlRuleTree"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             xmlns:local="clr-namespace:RulesEngine;assembly=RulesEngine"
             mc:Ignorable="d" d:DesignHeight="217" d:DesignWidth="232">

    <UserControl.Resources>

        <HierarchicalDataTemplate DataType="{x:Type local:RuleGroup}"  
                                  ItemsSource="{Binding Path=Rules}">
            
            <StackPanel Orientation="Horizontal">
                <TextBlock Margin="0,0,5,0" FontWeight="Bold">Group:</TextBlock>
                <TextBlock Text="{Binding Path=GroupName}"/>
            </StackPanel>              

        </HierarchicalDataTemplate>
.
.
.

You can see that 'local' is a reference to the other project. The error is on "HierarchicalDataTemplate DataType="{x:Type local:RuleGroup}"

When I hit "local:", intellisense shows me tje classes in the referenced project, so I know the ref works. Anyone see what I'm doing wrong?