IsPressed Trigger (Button) cause Exception - wpf

Question

I'm trying to override the button style.
Actually I've done it dozens of times, but now I came across this:
Exception and InnerException:
{"A property can not be null in Trigger."}
My Code:
<Style x:Key="ArrowRightStyle" TargetType="{x:Type Button}">
<Setter Property="FocusVisualStyle" Value="{StaticResource ButtonFocusVisual}"/>
<Setter Property="Background" Value="{StaticResource ButtonNormalBackground}"/>
<Setter Property="BorderBrush" Value="{StaticResource ButtonNormalBorder}"/>
<Setter Property="BorderThickness" Value="0"/>
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
<Setter Property="HorizontalContentAlignment" Value="Center"/>
<Setter Property="VerticalContentAlignment" Value="Center"/>
<Setter Property="Padding" Value="1"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate>
<Grid Height="{Binding ElementName=imgBackground, Path=ActualHeight, Mode=OneWay}"
Width="{Binding ElementName=imgBackground, Path=ActualWidth, Mode=OneWay}">
<Image x:Name="imgBackground" Source="{StaticResource RightArrowImageNormal}" Stretch="None"/>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsEnabled" Value="False">
<Setter TargetName="imgBackground" Property="Source" Value="{StaticResource RightArrowImageDisabled}"/>
</Trigger>
<Trigger Property="IsPressed" Value="True">
<Setter TargetName="imgBackground" Property="Source" Value="{StaticResource RightArrowImageIsPressed}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
and so it works:
<Style x:Key="ArrowRightStyle" TargetType="{x:Type Button}">
<Setter Property="FocusVisualStyle" Value="{StaticResource ButtonFocusVisual}"/>
<Setter Property="Background" Value="{StaticResource ButtonNormalBackground}"/>
<Setter Property="BorderBrush" Value="{StaticResource ButtonNormalBorder}"/>
<Setter Property="BorderThickness" Value="0"/>
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
<Setter Property="HorizontalContentAlignment" Value="Center"/>
<Setter Property="VerticalContentAlignment" Value="Center"/>
<Setter Property="Padding" Value="1"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate>
<Grid Height="{Binding ElementName=imgBackground, Path=ActualHeight, Mode=OneWay}"
Width="{Binding ElementName=imgBackground, Path=ActualWidth, Mode=OneWay}">
<Image x:Name="imgBackground" Source="{StaticResource RightArrowImageNormal}" Stretch="None"/>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsEnabled" Value="False">
<Setter TargetName="imgBackground" Property="Source" Value="{StaticResource RightArrowImageDisabled}"/>
</Trigger>
<DataTrigger Binding="{Binding RelativeSource={RelativeSource Mode=Self}, Path=IsPressed}" Value="True">
<Setter TargetName="imgBackground"
Property="Source" Value="{StaticResource RightArrowImageIsPressed}"/>
</DataTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
Why????

Answer 1

Just from quick look, I think it can't find the property IsPressed
Add "Button." should do it.
....
<Trigger Property="Button.IsPressed" Value="True">
<Setter TargetName="imgBackground" Property="Source" Value="{StaticResource....
or make sure that your template targets a button (I know you specified that on the style, but that ain't enough - The template will assume typeof(Control) and control doesn't have IsPressed on it.
...
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Button}">
....the rest of your code
I think that
{Binding RelativeSource={RelativeSource Mode=Self}, Path=IsPressed}"
resolves the relative source's type at run time as "Button",hence able to find IsPressed...