Windows RT 8.1, Windows 8.1, and Windows Server 2012 R2 Update: April 2014
Windows RT 8.1, Windows 8.1, and Windows Server 2012 R2 Update is a cumulative update that includes all previous released security updates and nonsecurity updates. In addition to previous updates, it includes improvements such as improved Internet Explorer 11 compatibility for enterprise applications, usability improvements, extended mobile device management and improved hardware support. http://support.microsoft.com/default.aspx?scid=kb;en-us;2919355
Important All future security and nonsecurity updates for Windows RT 8.1, Windows 8.1, and Windows Server 2012 R2 require this update to be installed. We recommend that you install this update on your Windows RT 8.1, Windows 8.1, or Windows Server 2012 R2-based computer in order to receive continued future updates. http://windows.microsoft.com/en-us/windows-8/install-latest-update-windows-8-1
In other words you must have Windows 8.1 Update 1 to get further updates from Microsoft!