Converter.Convert not being called

I am using VS2008. I am using a style trigger to change the background and tooltip of a textbox when there is an error. The Setter.Value for Property="ToolTip" used to be

<Binding RelativeSource="{x:Static RelativeSource.Self}" Path="(Validation.Errors)[0].ErrorContent" />

It basically worked but I was getting debugging errors because Validation.Errors would be empty. I tried this:

<Binding RelativeSource="{x:Static RelativeSource.Self}" Path="Validation.Errors" Converter="{StaticResource ves2obj}" />

The Convert method is not being called. Does anyone see what I am doing wrong?

Bill Swartz