XamlParseException: Attribute in custom control missing, but it's defined!

{System.Windows.Markup.XamlParseException: Unknown attribute Points on element SectionClickableArea. [Line: 10 Position: 16]
   at System.Windows.Application.LoadComponent(Object component, Uri resourceLocator)
   at SomeMainDialog.InitializeComponent()
   at SomeMainDialog..ctor()}

<l:SectionClickableArea x:Name="SomeButton"
    Points="528,350, 508,265, 520,195, 515,190, 517,165, 530,120, 555,75, 570,61, 580,60, 600,66, 615,80, 617,335, 588,395, 550,385, 540,390, 525,393, 520,385"
    Click="SomeButton_Click"/>

public partial class SectionClickableArea : Button {

	public static readonly DependencyProperty PointsProperty
		= DependencyProperty.Register("Points", typeof(PointCollection), typeof(SectionClickableArea),
			new PropertyMetadata((s, e) => {
				SectionClickableArea area = (SectionClickableArea) s;
				area.areaInfo.Points = (PointCollection) e.NewValue;
				area.UpdateLabelPosition();
			}));
	public PointCollection Points {
		get { return (PointCollection) GetValue(PointsProperty); }
		set { SetValue(PointsProperty, value); }
	}

Type: System.InvalidCastException Message: The object of type System.Windows.Controls.ContentControl could not be converted to type [...]SectionClickableArea. at SomeOtherMainDialog.InitializeComponent()
    at SomeOtherMainDialog..ctor()

Type: System.Exception Message:  An HRESULT E_FAIL error was returned when calling COM component at MS.Internal.XcpImports.CheckHResult(UInt32 hr)
    at MS.Internal.XcpImports.SetValue(INativeCoreTypeWrapper obj, DependencyProperty property, DependencyObject doh)
    at MS.Internal.XcpImports.SetValue(INativeCoreTypeWrapper doh, DependencyProperty property, Object obj)
    at System.Windows.DependencyObject.SetObjectValueToCore(DependencyProperty dp, Object value)
    at System.Windows.DependencyObject.SetValueInternal(DependencyProperty dp, Object value, Boolean allowReadOnlySet, Boolean isSetByStyle, Boolean isSetByBuiltInStyle)
    at System.Windows.DependencyObject.SetValueInternal(DependencyProperty dp, Object value)
    at System.Windows.DependencyObject.SetValue(DependencyProperty dp, Object value)
    at System.Windows.Controls.Control.set_DefaultStyleKey(Object value)
    at System.Windows.Controls.ContentControl..ctor()
    at System.Windows.CoreTypes.GetCoreWrapper(Int32 typeId)
    at MS.Internal.ManagedPeerTable.EnsureManagedPeer(IntPtr unmanagedPointer, Int32 typeIndex, Type type, Boolean preserveManagedObjectReference)
    at MS.Internal.ManagedPeerTable.EnsureManagedPeer(IntPtr unmanagedPointer, Int32 typeIndex, Type type)
    at MS.Internal.ManagedPeerTable.GetManagedPeer(IntPtr nativeObject)
    at MS.Internal.FrameworkCallbacks.SetPropertyAttribute(IntPtr nativeTarget, String attrName, String attrValue, String attachedDPOwnerNamespace, String attachedDPOwnerAssembly)

<UserControl.Resources>
   <customNamespace:InheritedControl x:Name="dummyInstance"/>
</UserControl.Resources>