Windows 2012 Expanding Cluster Volume Storage - Cluster Limit Failure

OS is Windows 2012 and I am trying to extend a cluster volume storage partition from 10 TB to 20 TB.  I have the LUN at 20 TB and Windows sees the 20 TB disk in both Disk Management and in Failover Cluster Manager -> Storage -> Disks.

I am following the procedure in the this TechNet article:
http://blogs.technet.com/b/chrad/archive/2010/07/15/cluster-shared-volumes-csv-extending-a-volume.aspx

When I run the extend command in DiskPart I get the following error:

Virtual Disk Service error:
The volume cannot be extended because the number of clusters will
exceed the maximum number of clusters supported by the file system.

I also get this same error when I try to extend the drive through Disk Management.

My disk partition is GPT so I should have a 256 TB storage limit.  

I've Googled around but could not find anything related to this issue for 2012.

Any suggestions on how to successfully extend my Cluster Storage Volume?

Thanks,

Al