How to change the message in validation error

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Silverlight

Hi all,

I have a silverlight application in which i am doing validation my code is like this

<toolkit:DataField IsRequired="True" Label="Incident Type" VerticalAlignment="Center" >
                           <SIMSTagComboBox VerticalAlignment="Center" HorizontalAlignment="Left" Height="25" Width="220"  TagTypeID="9" SelectedValue="{Binding Guid1, Mode=TwoWay}"   />
                       </toolkit:DataField>

In this when I left the incident Type field as empty I am getting error message as "Input string is not in correct format" .I want to customize this message as "Please enter the value in Incident type".Please provide some help.

Regards