Cannot add content of type 'System.Windows.Interactivity.EventTrigger' to an object of type 'System.Windows.Interactivity.TriggerCollection'.

Hi All,

Recently I changed a .Net 4.0 WPF project to .Net 3.5 . My code is:

<i:Interaction.Triggers>
                <i:EventTrigger EventName="SelectionChanged">
                    <i:InvokeCommandAction CommandName="SelectionChangedCommand" 
                                           CommandParameter="{Binding Path=SelectedItems, ElementName=instancesDataGrid}"/>
                </i:EventTrigger>
            </i:Interaction.Triggers>

and I reference the namespace:

xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity" 

I can compile the project successfully. But while running the programe I get an error which is "Cannot add content of type 'System.Windows.Interactivity.EventTrigger' to an object of type 'System.Windows.Interactivity.TriggerCollection'. ". Any ideas? Thanks.