Commented Unassigned: SliderExtender produces a JavaScript error [27936]

The following markup previously worked without issue before I upgraded to the devexpress managed version of the toolkit (v15.1.2 and recently v15.1.3)

<asp:TextBox runat="server" ID="txtSlider"></asp:TextBox>
<asp:TextBox runat="server" ID="txtSliderValue"></asp:TextBox> %
<asp:SliderExtender runat="server" ID="sliderExtender"
TargetControlID="txtSlider" BoundControlID="txtSliderValue"
Minimum="0" Maximum="100" Steps="100"/>

Since the upgrade, I now get the following java script error thrown and the slider doesn't render

__Error: Unable to get property 'Slider.Handle-Horizontal.gif' of undefined or null reference__

The error is thrown in ScriptResource.axd line 117

var defaultHandleImageUrl = (this._isHorizontal)
? Sys.Extended.UI.Images["Slider.Handle-Horizontal.gif"]
: Sys.Extended.UI.Images["Slider.Handle-Vertical.gif"];

Sys.Extended.UI exists but Sys.Extended.UI.Images is undefined

What has changed that has broken this int he new versions?
Comments: The title was edited as this issue is included in What's new list for the v15.1.4 release.