How to reset the value of the property BackgroundGradientStops in VisualStateManager?

[ArtemProg]

The code leads to an error:
<Setter Property="BackgroundGradientStops" Value="{x:Null}" />

This code works:

<Setter Property="BackgroundGradientStops">
	<Setter.Value>
		<yummy:GradientStopCollection/>
	</Setter.Value>
</Setter>

Full example with error:

<yummy:PancakeView>

	<VisualStateManager.VisualStateGroups>
		<VisualStateGroup x:Name="CommonStates">
			
			<VisualState x:Name="Normal">
				<VisualState.Setters>
					<Setter Property="BackgroundGradientStops" Value="{x:Null}" />
				</VisualState.Setters>
			</VisualState>
			
			<VisualState x:Name="Selected">
				<VisualState.Setters>
					<Setter Property="BackgroundGradientStartPoint" Value="0,0" />
					<Setter Property="BackgroundGradientEndPoint" Value="1,0" />
					<Setter Property="BackgroundGradientStops">
						<Setter.Value>
							<yummy:GradientStopCollection>
								<yummy:GradientStop Color="#ff4157" Offset="0" />
								<yummy:GradientStop Color="#ff4359" Offset="0.5" />
								<yummy:GradientStop Color="#fe93c5" Offset="1" />
							</yummy:GradientStopCollection>
						</Setter.Value>
					</Setter>
				</VisualState.Setters>
			</VisualState>
			
		</VisualStateGroup>
	</VisualStateManager.VisualStateGroups>
	
</yummy:PancakeView>

Tell me why you can not use Value = "{x: null}"?