ComboBox throws an exception

I have a combo obx that is throwing a exception.

The error in the build says: Provide value on 'System.Windows.Markup.StaticResourceHolder' threw an exception"

<ComboBox
                    Name="plotCombo"
                    HorizontalAlignment="Left"
                    VerticalAlignment="Center"
                    Height="25"
                    SelectedValue="{Binding Path=SelectedOption, Mode=TwoWay}"></ComboBox>

chuckdawit