I have the following xaml for changing the image for WPF button when mouse is on the button. It give below error. Any help is appreciated...

'System.Windows.Media.Animation.DoubleAnimation' animation object cannot be used to animate property 'Source' because it is of incompatible type 'System.Windows.Media.ImageSource'.

<Style TargetType="{x:Type local:ButtonControl}">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type local:ButtonControl}">
                    <Grid>
                        <VisualStateManager.VisualStateGroups>
                            <VisualStateGroup x:Name="CommonStates">
                                <VisualState x:Name="MouseOver">
                                    <Storyboard>
                                        <DoubleAnimation 
                                                Storyboard.TargetName="img"
                                                Storyboard.TargetProperty="Source"
                                                To="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=MouseOverImage}"
                                                />
                                    </Storyboard>
                            </VisualStateGroup>
                        </VisualStateManager.VisualStateGroups>
                            <Border>
                                <Image x:Name="img"
                                         Source="pack://application:,,,/Recipe_06_13;component/Resources/normal.bmp"
                                />
                            </Border>
                    </Grid>
                </ControlTemplate>
        </Setter.Value>
    </Setter>
    </Style>