Error Expanding Large VHD

I am getting an error expanding a 14 TB vhd to 18 TB. The server is 2008 R2 SP1 with all patches applied. Here are the commands and error below.

C:\Users\Administrator.SWRI18>diskpart

Microsoft DiskPart version 6.1.7601
Copyright (C) 1999-2008 Microsoft Corporation.
On computer: DIV18HYPERV4

DISKPART> Select vdisk file="C:\ClusterStorage\Volume2\Hyper-V\div18fs1\Virtual Hard Disks\share1.vhd"

DiskPart successfully selected the virtual disk file.

DISKPART> list vdisk

  VDisk ###  Disk ###  State                 Type       File
  ---------  --------  --------------------  ---------  ----
* VDisk 0    Disk ---  Added                 Unknown     C:\ClusterStorage\Volum
e2\Hyper-V\div18fs1\Virtual Hard Disks\share1.vhd

DISKPART> expand vdisk maximum=18500000

  100 percent completed
DiskPart has encountered an error: The parameter is incorrect.
See the System Event Log for more information.