virtualbox - VBOX_E_NOT_SUPPORTED on Dynamically Sized .vdi

I'm trying to resize a VM using the following:

$ VBoxManage modifyhd ff2d9746-0c51-42f6-995b-611e0b3bcc05 --resize 15000
  0%...
  Progress state: VBOX_E_NOT_SUPPORTED
  VBoxManage: error: Resize hard disk operation for this format is not implemented yet!

However, everything I can find about this error points to either not using a .vdi or not using a dynamically sized .vdi.

I am

UUID:           ff2d9746-0c51-42f6-995b-611e0b3bcc05
Parent UUID:    base
State:          created
Type:           normal (base)
Location:       /home/[username]/VirtualBox VMs/Server2/Server2.vdi
Storage format: VDI
Capacity:       15360 MBytes

and

$ VBoxManage showhdinfo 5dba2121-5dbf-4945-85e1-57aa66f7295d
  UUID:           5dba2121-5dbf-4945-85e1-57aa66f7295d
  Parent UUID:    base
  State:          created
  Type:           normal (base)
  Location:       /home/[username]/VirtualBox VMs/Server/Server/Server.vdi
  Storage format: VDI
  Format variant: dynamic default
  Capacity:       15360 MBytes
  Size on disk:   13218 MBytes
  In use by VMs:  SomeServer (UUID: d1a2a616-f2ff-4d95-9ade-e44c960d74b7)

Running the following version:

$ vboxmanage --version
  4.3.18r96516

Answer

The problem ended up that I was trying to shrink the drive instead of increase it. The --resize amount is the final size, and is not added to the current size.

VBoxManage modifyhd ff2d9746-0c51-42f6-995b-611e0b3bcc05 --resize 30000