Created Unassigned: SliderExtender JavaScript error [28004]

Platforms
.Net 4.0
ACT Version 15.1.4.0 (and previously the 15.1.1.100 version)

The project exhibiting the problem is using Master/Child page configuration.

The Master page contains the ScriptManager, simply declared as:
```
<asp:ScriptManager ID="scriptmanager1" runat="server" />
```
The ACT is declared for the project within the web.config:
```
<pages>
<controls>
<add tagPrefix="ajaxToolkit" assembly="AjaxControlToolkit" namespace="AjaxControlToolkit" />
</controls>
</pages>
```
When using the SliderExtender control within a child page, and using custom graphics for the slider (_specifically_, setting the __RailCssClass__ property), we see the following error:

__Sys.ArgumentException: Please set valid values for the height and width attributes in the slider's CSS classes__

When using the same control extender and custom rail within a different project (the only difference being it doesn't use the Master/Child page model), the control works correctly and as expected.

If I remove the RailCssClass property, the error doesn't occur (but I don't get the custom graphic of course).