xmalparserexception

Question

Hi all, I have the below in my usercontrol

 <ToggleButton x:Name="HeaderSite" Grid.RowSpan="2" Margin="0" MinWidth="0" MinHeight="0" 
                                                  Style="{StaticResource Expander.DownHeaderStyle}" FocusVisualStyle="{StaticResource DefaultFocusVisualStyle}"                                       
                                                  IsChecked="{Binding Path=IsExpanded,Mode=TwoWay,RelativeSource={RelativeSource TemplatedParent}}"
                                                    Content="{TemplateBinding Header}" ContentTemplate="{TemplateBinding HeaderTemplate}" ContentTemplateSelector="{TemplateBinding HeaderTemplateSelector}" Foreground="{TemplateBinding Foreground}" Padding="{TemplateBinding Padding}"
                                                    FontFamily="{TemplateBinding FontFamily}" FontSize="{TemplateBinding FontSize}" FontStyle="{TemplateBinding FontStyle}" FontStretch="{TemplateBinding FontStretch}" FontWeight="{TemplateBinding FontWeight}" HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}" VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"/>

I have placed this control in a tabitem .Whenever i switch between tabs in the tabcontrol,i get the below error XAML parser exception.Resource Key not found.Value cannot be null.

Why is the tab not recognising the static resources on the tab items. ?